Disagree with; act against (6)
I believe the answer is:
oppose
This clue is a double definition.
'disagree with' is the first definition.
(thesaurus)
'act against' is the second definition.
(I've seen this in another clue)
(Other definitions for oppose that I've seen before include "Take stand against, resist" , "Obstruct, resist" , "Act against" , "Compete with" , "Argue against" .)