Threaten troublemaker, the limit (6)
I believe the answer is:
impend
'threaten' is the definition.
(thesaurus)
'troublemaker the limit' is the wordplay.
'troublemaker' becomes 'imp' (I've seen this before).
'the limit' becomes 'end' (I've seen this before).
'imp'+'end'='IMPEND'
(Other definitions for impend that I've seen before include "(Of a bad thing) loom" , "Become imminent" , "appearing to be imminent" , "Be imminent or about to happen" , "Be looming" .)