Back boxer's assistant (6)
I believe the answer is:
second
I believe this clue is a double definition.
'back' is the first definition.
('second' can be a synonym of 'back')
'boxer's assistant' is the second definition.
(I've seen this before)
(Other definitions for second that I've seen before include "Attendant in a duel or boxing match" , "honours degree" , "Short period of time" , "Extra" , "taking silver back" .)