Cunning wearing the man's suit (6)
I believe the answer is:
hearts
'suit' is the definition.
(hearts is a playing card suit)
'cunning wearing the man's' is the wordplay.
'cunning' becomes 'art' (both can mean artfulness or trickery).
'wearing' means one lot of letters goes inside another.
'the man's' becomes 'hes' ('he is' means a particular man is).
'art' placed within 'hes' is 'HEARTS'.
(Other definitions for hearts that I've seen before include "Soccer club" , "Red playing cards" , "Suit for vital organs" , "Suit in pack" , "See 23" .)