Lady’s crazy about ecstasy (4)
I believe the answer is:
dame
'lady's' is the definition.
(synonyms)
'crazy about ecstasy' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'about' says the letters should be written in reverse.
'ecstasy' becomes 'e' (informal term for the drug).
'mad' written backwards gives 'dam'.
'dam'+'e'='DAME'
(Other definitions for dame that I've seen before include "Broad" , "Crossdressed panto role" , "Woman" , "Lady's title" , "Comic pantomime role" .)