Sarcastic chap supports double entendre (7)
I believe the answer is:
pungent
'sarcastic' is the definition.
('pungent' can be similar in meaning to 'sarcastic')
'chap supports double entendre' is the wordplay.
'chap' becomes 'gent' (I've seen this in other clues**).
'supports' is a charade indicator (letters next to each other) (bottom letters provide a support for the top ones).
'double entendre' becomes 'pun' (I've seen this before).
'gent' after 'pun' is 'PUNGENT'.
(Other definitions for pungent that I've seen before include "spicy" , "Witty" , "bitter" , "Biting" , "Penetrating" .)