Initially, each gain only serves for self (3)
I believe the answer is:
ego
'for self' is the definition.
(synonyms)
'initially each gain only' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'each gain only' is 'EGO'.
'serves' acts as a link.
(Other definitions for ego that I've seen before include "Self-esteem, sense of self" , "Selfishness" , "See 21 Down" , "Person's sense of self" , "Part of the mind" .)