Unfit male's horrible rage (9)
I believe the answer is:
fulminate
'rage' is the definition.
(thesaurus)
'unfit male's horrible' is the wordplay.
'horrible' is an anagram indicator.
'unfit'+'male'='unfitmale'
'unfitmale' with letters rearranged gives 'FULMINATE'.
(Other definitions for fulminate that I've seen before include "blow out" , "Explode with anger, criticism" , "Protest loudly and bitterly" , "Rail" .)