In war I valiantly seek adversary (5)
I believe the answer is:
rival
'adversary' is the definition.
(thesaurus)
'in war i valiantly seek' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RIVAL' is hidden in the letters of 'war i valiantly seek'.
(Other definitions for rival that I've seen before include "Contender" , "Compete with" , "Emulate" , "Contestant you hope to defeat" , "Someone you hope to defeat" .)