Opposed to, slangily (4)
I believe the answer is:
agin
'opposed to slangily' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for agin that I've seen before include "Opposed to local" , "Against (facetious)" , "Not in favour of" , "Not a fan of" , "Hostile to" .)