One who helps a superior it's said (7)
I believe the answer is:
abettor
'one who helps' is the definition.
(I've seen this before)
'a superior it's said' is the wordplay.
'superior' becomes 'better' (better is a kind of superior).
'it's said' indicates a 'sounds like' (homophone) clue.
'better' is a homophone of 'bettor'.
'a'+'bettor'='ABETTOR'
(Other definitions for abettor that I've seen before include "Accomplice" , "Assistant" , "fellow-criminal" , "Associate" , "One aids another" .)