Cunning insult overheard (7)
I believe the answer is:
sleight
'cunning' is the definition.
(I've seen this before)
'insult overheard' is the wordplay.
'insult' becomes ''slight' (I've seen this before).
'overheard' indicates a 'sounds like' (homophone) clue.
'slight' is a homophone of 'SLEIGHT'.
(Other definitions for sleight that I've seen before include "Skilful deception" , "Conjuror uses this" , "Dexterity (usually of hand)" , "distraction?" , "Manual dexterity" .)