Threaten troublemaker with ruin (6)
I believe the answer is:
impend
'threaten' is the definition.
(I've seen this before)
'troublemaker with ruin' is the wordplay.
'troublemaker' becomes 'imp' (I've seen this before).
'with' says to put letters next to each other.
'ruin' becomes 'end' (ruin something means to end it).
'imp'+'end'='IMPEND'
(Other definitions for impend that I've seen before include "Be looming" , "appearing to be imminent" , "Threaten; approach" , "Be imminent or about to happen" , "(Of a bad thing) loom" .)