Deny a wild brute (5)
I believe the answer is:
rebut
'deny' is the definition.
(to rebut is to deny or refute)
'a wild brute' is the wordplay.
'a wild' is an anagram indicator.
'brute' with letters rearranged gives 'REBUT'.
(Other definitions for rebut that I've seen before include "Disprove an accusation" , "Give the lie to" , "Scotch" , "Tuber (anag.)" , "Refute - disprove" .)