Demon with extreme menace (6)
I believe the answer is:
impend
'menace' is the definition.
(thesaurus)
'demon with extreme' is the wordplay.
'demon' becomes 'imp' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'extreme' becomes 'end' (I've seen this before).
'imp'+'end'='IMPEND'
(Other definitions for impend that I've seen before include "Be imminent or about to happen" , "Threaten; approach" , "(Of a bad thing) loom" , "appearing to be imminent" , "seen to be around the corner" .)