It turns up after an opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(someone opposed to something)
'it turns up after an' is the wordplay.
'turns up' is a reversal indicator.
'after' means one lot of letters go next to another.
'it' written backwards gives 'ti'.
'ti' put after 'an' is 'ANTI'.
(Other definitions for anti that I've seen before include "[NO MAN]" , "on the other side" , "Prefix meaning against" , "one opposing?" , "(Person who is) not in favour" .)