Testament maintained by perfect opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(I've seen this before)
'testament maintained by perfect' is the wordplay.
'testament' becomes 'nt'.
'maintained by' is an insertion indicator (maintain can mean to keep hold of).
'perfect' becomes 'ai' (I've seen this in other clues).
'nt' inserted within 'ai' is 'ANTI'.
(Other definitions for anti that I've seen before include "Someone opposed (to a policy?)" , "Person opposed" , "Naysayer" , "Negative vote" , "'No' voter" .)