Evil scoundrels on ecstasy (5)
I believe the answer is:
curse
'evil' is the definition.
(I've seen this before)
'scoundrels on ecstasy' is the wordplay.
'scoundrels' becomes 'curs'.
'on' means one lot of letters go next to another.
'ecstasy' becomes 'e' (informal term for the drug).
'curs'+'e'='CURSE'
(Other definitions for curse that I've seen before include "Imprecation or oath" , "Blasphemy" , "Blaspheme; damn" , "Excommunication" , "Profane or obscene expression" .)