You don't say recover having swallowed ecstasy (6)
I believe the answer is:
really
'you don't say' is the definition.
(both can express surprise)
'recover having swallowed ecstasy' is the wordplay.
'recover' becomes 'rally' (rallying is a kind of recovering).
'having swallowed' indicates putting letters inside.
'ecstasy' becomes 'E' (informal term for the drug).
'rally' placed around 'e' is 'REALLY'.
(Other definitions for really that I've seen before include "Honestly, it's a fact" , "In actual fact" , "Truly, in fact" , "surprised expression" , "Actually, honest to God" .)