Threaten demon with death (6)
I believe the answer is:
impend
'threaten' is the definition.
(thesaurus)
'demon with death' is the wordplay.
'demon' becomes 'imp' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'death' becomes 'end' ('end' can be a synonym of 'death').
'imp'+'end'='IMPEND'
(Other definitions for impend that I've seen before include "Be looming" , "Be about to take place" , "Be imminent or about to happen" , "appearing to be imminent" , "(Of a bad thing) loom" .)