Let’s look at important things here –
Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
Dhanishtha Nakshatra - It is a nakshatra related with wealth, luxury and lavish life. It is also a nakshatra related with music. Also, it is a nakshatra of celibacy. Please check this link for more info on Dhanishtha Nakshatra - https://www.astrosaxena.com/Dhannaksh.
Capricorn - As Dhanishtha is part of Capricorn sign, Capricorn and things represented by Capricorn are also important here. Capricorn is the 10th sign of zodiac belt, hence it represents many things related with 10th house of horoscope like Law and Order, Government, Authority, Organisation, Discipline etc. Besides it also represents World outside your home and people, Fear and Anxiety etc. Capricorn is consisted of next 2 and half Nakshatra, i.e. Uttar-Ashadha, Shravana and Dhanishtha. Capricorn is ruled by Saturn and that's why it represents Fear and Anxiety.
Aquarius - As Dhanishtha is also a part of Aquarius sign, Aquarius and things represented by Aquarius are also important here. Aquarius is 11th sign of zodiac belt, hence it signifies the things and energy related with 11th house of horoscope, such as Gains, Desires, Hopes and Wishes, Large Organisations, Network Circles etc. Besides this, Aquarius also represents Scientific Thinking & Research, Uplifting the Society, Higher Goals & Rewards for all. Aquarius is made of another set of 2 and half Nakshatra, i.e. Dhanishtha, Satabhisha and Purva-Bhadrapada. Aquarius Lord is Saturn & Rahu.
Mars - As Mars rules Dhanishtha, Mars's position and dignity is important to know about the overall functioning of any planet in Dhanishtha.
Saturn & Rahu - As Saturn rules Capricorn & Aquarius and Rahu co-rules Aquarius, their position and dignity is important to know about the overall functioning of any planet in Dhanishtha/Capricorn and Dhanishtha/Aquarius.
Houses Involved – Also, we can’t afford to forget the houses involved as a planet can be ruling different houses and sitting in different house for every person as per ascendant and planet’s position. To know overall affect, we have to consider the houses involved as those areas of life are being impacted.
Interpretation of Mercury in Dhanishtha Nakshatra – Here, Mercury is in friendly signs ruled by Saturn and in enemy nakshatra ruled by Mars. So again, it is an average type of position of Mercury, neither too bad nor too good. As Dhanishtha is quite clearly a nakshatra of finance, wealth & assets and Mercury represents business, these people can be involved in any business related to finance or assets related matters. In Capricorn section, they can be looking for the authority and recognition with this kind of work whereas in Aquarius section, the purpose can be serving humanity and entrepreneurship. In Capricorn section, they can be a Lawyer too in financial nature of litigation. Even if they are not in financial business or consulting, they can still be very good with financial dealings.
At the same time, Dhanishtha is nakshatra of Celibacy and Marital Conflicts. So, if Mercury is a relationship oriented planet, person can find lots of problem in maintaining a balanced and meaningful relation. So, in relationship matters, person needs to show lots of maturity to sustain it. It can even make them relationship counsellor or divorce lawyer.
Role of Mars – Mars placement is important to guide Mercury in right direction, being Dhanishtha Lord.
Role of Saturn & Rahu - Also, Saturn-Rahu placement is important to guide Mars in right direction, being Capricorn & Aquarius lords.
Other Factors – And of course, we need to consider conjunction, aspects, house lordship, house position and dashas etc to know the exact results of Mercury in Dhanishtha. So, we always need to take a holistic view of chart rather than just 1 planetary position.
Conclusion – So, this is how I see Mercury in Dhanishtha can work in a chart.