I try to follow enthusiastically at the beginning (3)
I believe the answer is:
ego
'i' is the definition.
('I' in Latin)
'try to follow enthusiastically at the beginning' is the wordplay.
'try' becomes 'go' (both can mean an attempt).
'to follow' means one lot of letters go next to another (some letters go after or follow others).
'at the beginning' indicates taking the first letters.
The first letter of 'enthusiastically' is 'e'.
'go' put after 'e' is 'EGO'.
(Other definitions for ego that I've seen before include "See 21 Down" , "Self-confidence" , "Self-esteem, conceit" , "(Self-assertive) personality" , "Personal pride" .)