Fight back, and sit on work (6)
I believe the answer is:
oppose
'fight back' is the definition.
('oppose' can be a synonym of 'fight back')
'sit on work' is the wordplay.
'sit' becomes 'pose' ('pose' can be a synonym of 'sit').
'on' is a charade indicator (letters next to each other).
'work' becomes 'op' (abbreviation for opus).
'pose' after 'op' is 'OPPOSE'.
'and' acts as a link.
(Other definitions for oppose that I've seen before include "match" , "Act against" , "Take stand against, resist" , "'Refute, stand against (6)'" , "Counter" .)