OK to accommodate extremely evil cat (6)
I believe the answer is:
feline
'cat' is the definition.
(I've seen this before)
'ok to accommodate extremely evil' is the wordplay.
'ok' becomes 'fine' (synonyms).
'to accommodate' means one lot of letters goes inside another.
'extremely' suggests removing the centre (only the extremes of the word are used).
'evil' with its middle taken out is 'el'.
'fine' going around 'el' is 'FELINE'.
(Other definitions for feline that I've seen before include "Cat-like" , "Of or like cats" , "Mog" , "Persian for one" , "16 [TOM] perhaps" .)