Strive; quarrel (5)
I believe the answer is:
fight
I believe this clue is a double definition.
'strive' is the first definition.
(thesaurus)
'quarrel' is the second definition.
(I've seen this in another clue)
(Other definitions for fight that I've seen before include "Spar" , "Brawl; argue" , "Struggle; quarrel" , "(Take part in) battle" , "Contest" .)