A superior assistant (7)
I believe the answer is:
abetter
'assistant' is the definition.
(I've seen this before)
'a superior' is the wordplay.
'superior' becomes 'better' (better is a kind of superior).
'a'+'better'='ABETTER'
(Other definitions for abetter that I've seen before include "who gives encouragement?" , "One who encourages" , "accessory" , "One who illegally helps" , "Partner in crime; tree bat (anag.)" .)