First enjoying success, it all evens out (7)
I believe the answer is:
initial
'first' is the definition.
(similar in meaning)
'enjoying success it all evens out' is the wordplay.
'enjoying success' becomes 'in it' (I've seen this before).
'evens out' indicates alternate letters (remove letters in even-numbered positions).
'it'+'all'='itall'
The alternate letters of 'itall' are 'ial'.
'init'+'ial'='INITIAL'
(Other definitions for initial that I've seen before include "Occurring at the start" , "First letter of a name" , "First - opening" , "Endorse" , "First letter (of a word)" .)