Hellish cat is an oddball (7)
I believe the answer is:
satanic
'hellish' is the definition.
(synonyms)
'cat is an oddball' is the wordplay.
'oddball' indicates an anagram.
'cat'+'is'+'an'='catisan'
'catisan' is an anagram of 'SATANIC'.
(Other definitions for satanic that I've seen before include "Nick's" , "Devilish (as Blake's 18)" , "like Mephisto" , "Bad" , "Relating to the devil" .)