Ecstasy hidden in French wine vessel (4)
I believe the answer is:
vein
'vessel' is the definition.
(vein is a kind of vessel)
'ecstasy hidden in french wine' is the wordplay.
'ecstasy' becomes 'e' (informal term for the drug).
'hidden in' is an insertion indicator.
'french wine' becomes 'vin' (I've seen this before).
'e' inserted into 'vin' is 'VEIN'.
(Other definitions for vein that I've seen before include "Distinctive quality" , "Seam of mineral" , "Tube that carries blood to the heart" , "part of body" , "Blood-carrying vessel" .)