Stand firm against (6)
I believe the answer is:
resist
'stand firm against' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for resist that I've seen before include "Sister (anag)" , "Withstand or defy" , "Oppose or fight against someone" , "Put up opposition, defy" , "Put up fight, oppose forcefully" .)