Intransigent troublemaker left a message (10)
I believe the answer is:
implacable
'intransigent' is the definition.
(thesaurus)
'troublemaker left a message' is the wordplay.
'troublemaker' becomes 'imp' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'message' becomes 'cable' (cable is a kind of message**).
'imp'+'l'+'a'+'cable'='IMPLACABLE'
(Other definitions for implacable that I've seen before include "unable to be stopped" , "Unable to be appeased" , "Totally unrelenting" , "adamant" , "Relentless" .)