I mostly promote virtues of fully-honest leader (4)
I believe the answer is:
self
'i' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'mostly promote virtues of fully-honest leader' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'promote' becomes 'sell' (sell can mean to promote or advertise).
'virtues of fully honest leader' becomes 'f' (I can't justify this - if you can you should give a lot more credence to this answer).
'sell' with its last letter removed is 'sel'.
'sel'+'f'='SELF'
Can you help me to learn more?
(Other definitions for self that I've seen before include "Will the author" , "One's individuality" , "Will perhaps" , "Personality" , "A person's awareness of their own identity" .)