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.
Uttara Phalguni - Uttara Phalguni is nakshatra of hard work, realistic approach and practicalities of life. It is also nakshatra of agreements, deals and conflicts. For detailed information, please check this - https://www.astrosaxena.com/uphnaksh.
Leo - As Uttara-Phalguni is a part of Leo, Leo sign and its representations become important. Leo is 5th sign of zodiac belt, hence it represents the things associated with 5th house of horoscope like Love, Creativity, Arts, Being on Stage etc. Besides that, Leo is basically the Lion. This is the sign of feeling like King or Queen. Also, it is a sign of independence, Confidence, Ego, Self Esteem etc. Leo is consisted of 2 and half Nakshatra namely Magha, Purva-Phalguni and Uttara-Phalguni. Leo's lord is Sun.
Virgo - As Uttara-Phalguni is also a part of Virgo, Virgo sign and its representations also become important. Virgo is the 6th sign of zodiac belt, hence it represents almost all the things which are represented by the 6th house of horoscope like Debts, Diseases, Obstacles, Serving the under-privileged, Service Life, etc. Besides this, Virgo represents analytical or critical skills of people, being perfectionist and healing abilities. Virgo's basic representation is the lady coming in boat with wheat and herbs in her hands. So, Virgo is basically a sign of healing. Virgo consists of next set of 2 and half nakshatra. i.e. Uttara-Phalguni, Hasta and Chitra. Virgo's lord is Mercury.
Sun - As Sun rules Leo and Uttara-Phalguni, Sun's position and dignity is important to know about the overall functioning of any planet in Uttara Phalguni/Leo.
Mercury - As Mercury rules Virgo, Mercury's position and dignity is important to know about the overall functioning of any planet in Uttara Phalguni/Virgo.
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 Uttara Phalguni Nakshatra – Mercury is in friendly nakshatra and friendly sign in Leo, as both are ruled by Sun. Then Mercury is exalted in Virgo section of this nakshatra. It means that Mercury is very strong in dignity in this nakshatra. As Uttara Phalguni represents financial dealings and Mercury is a very calculative business person, it shows that these people can be very good in financial dealings. They can be in work or business related with finances. They can be working in stock/share markets or in fields like accountancy. This result becomes even more potential when Mercury is in Virgo section of Uttara Phalguni as Mercury is exalted here. They can be really clever or almost cunning with their business or money matters. At the same time, they can be very meticulous with their work and they can be seen as Lawyers too as Virgo is sign of conflicts. They can be very clever in their communications. At the same time, this perfection or cleverness may not work in their favor in matters of relationship as none can be perfect or ideal for anyone else.
Role of Sun – Sun placement is important to guide Mercury in right direction, being Uttara Phalguni and Leo's lord.
Role of Mercury - As we are trying to interpret Mercury only, we need not to look at Mercury's placement separately.
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 Uttara Phalguni. 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 Uttara Phalguni can work in a chart.