Strive against (6)
I believe the answer is:
resist
'strive 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 "Put up fight, oppose forcefully" , "Withstand a force" , "Refuse to give in to" , "Stand firm against" , "Stand up to, fight against" .)