Woman has everyone back on MDMA (4)
I believe the answer is:
ella
'woman' is the definition.
(woman's name)
'everyone back on mdma' is the wordplay.
'everyone' becomes 'all' (all people).
'back' says the letters should be written backwards.
'on' is a charade indicator (letters next to each other).
'mdma' becomes 'E' (MDMA is ecstasy, ie 'E').
'all' reversed gives 'lla'.
'lla' after 'e' is 'ELLA'.
'has' acts as a link.
(Other definitions for ella that I've seen before include "---- Fitzgerald, 'Queen of Jazz'" , "Miss" , "Great late jazz singer . . . . Fitzgerald" , "Diminutive of Eleanor" , "Ms Fitzgerald" .)