Dead quickly takes ecstasy (2,5)
I believe the answer is:
at peace
'dead' is the definition.
(at peace means deceased)
'quickly takes ecstasy' is the wordplay.
'quickly' becomes 'at pace' (to do something at pace is to do it quickly).
'takes' means one lot of letters goes inside another.
'ecstasy' becomes 'E' (informal term for the drug).
'atpace' enclosing 'e' is 'AT PEACE'.
(Other definitions for at peace that I've seen before include "calm" , "Resting serenely" , "no longer with us" , "In a state of serenity" , "Not disputing" .)