Pure evil's monstrous, monstrous (9)
I believe the answer is:
repulsive
'monstrous' is the definition.
(repulsive can mean monstrous or horrible)
'pure evil's monstrous' is the wordplay.
'monstrous' indicates anagramming the letters.
'pure'+'evils'='pureevils'
'pureevils' anagrammed gives 'REPULSIVE'.
(Other definitions for repulsive that I've seen before include "Obnoxious" , "Offensive" , "ugly" , "Abhorrent" , "Disgusting" .)