Horrible: evil perhaps (4)
I believe the answer is:
vile
'horrible' is the definition.
(I've seen this before)
'evil perhaps' is the wordplay.
'perhaps' is an anagram indicator.
'evil' is an anagram of 'VILE'.
(Other definitions for vile that I've seen before include "Disgusting kind of evil" , "Very nasty, quite evil" , "Extremely unpleasant" , "Noxious" , "Loathsome - draw veil over it" .)