Mark rebel out, needing reform — me? (12)
I believe the answer is:
troublemaker
'me?' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's an association between them I don't understand?
'mark rebel out needing reform' is the wordplay.
'needing reform' indicates an anagram.
'mark'+'rebel'+'out'='markrebelout'
'markrebelout' is an anagram of 'TROUBLEMAKER'.
(Other definitions for troublemaker that I've seen before include "Incendiary" , "Person stirring up problems" , "Agitator" , "Ringleader" , "Stirrer" .)