Completely give up ecstasy (5)
I believe the answer is:
quite
'completely' is the definition.
(synonyms in phrases like 'are you quite done?')
'give up ecstasy' is the wordplay.
'give up' becomes 'quit' (both can mean to resign).
'ecstasy' becomes 'E' (informal term for the drug).
'quit'+'e'='QUITE'
(Other definitions for quite that I've seen before include "Completely or entirely - be quiet" , "Rather" , "Somewhat; completely" , "Somewhat; fully" , "Fairly; indeed" .)