I am uppermost in his thoughts (7)
I believe the answer is:
egotist
'am uppermost in his thoughts' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'i' is the wordplay.
I cannot really understand how this works, but
'i' could be 'ego' ('I' in Latin) and 'ego' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for egotist that I've seen before include "Self-interested person" , "Boaster" , "Person with inflated self-image" , "Self-obsessed person" , "What I want" .)