Sister prepared for combat (6)
I believe the answer is:
resist
'for combat' is the definition.
(I've seen this before)
'sister prepared' is the wordplay.
'prepared' indicates anagramming the letters.
'sister' is an anagram of 'RESIST'.
(Other definitions for resist that I've seen before include "'Withstand, fight against (6)'" , "Fight back" , "Hold out for sister" , "Withstand, struggle against" , "Put up fight, oppose forcefully" .)