Fight back (6)
I believe the answer is:
resist
'fight back' is the definition.
(I know that resist is a more specific form of fight back)
This is the entire clue.
(Other definitions for resist that I've seen before include "Use force to prevent something from happening to sister" , "Bar" , "Offer opposition to" , "'Withstand, fight against (6)'" , "Refuse to comply" .)