Look after little devil causing harm (6)
I believe the answer is:
impair
'harm' is the definition.
(thesaurus)
'look after little devil' is the wordplay.
'look' becomes 'air' (I've seen this before).
'after' is a charade indicator (letters next to each other).
'little devil' becomes 'imp' (I've seen this before).
'air' put after 'imp' is 'IMPAIR'.
'causing' acts as a link.
(Other definitions for impair that I've seen before include "Hurt or handicap" , "Hinder" , "blunt" , "Cause weakening of, damage" , "Harm" .)