Against or opposite (6)
I believe the answer is:
contra
This clue is a double definition.
'against' is the first definition.
(I've seen this in another clue)
'opposite' is the second definition.
(I've seen this in another clue)
(Other definitions for contra that I've seen before include "Revolutionary fighter" , "Irregular" , "US-backed fighter" , "on the other side" , "Con, Art could be against it" .)