Arch scoundrel opposed to ecstasy (5)
I believe the answer is:
curve
'arch' is the definition.
(I know that arch can be written as curve)
'scoundrel opposed to ecstasy' is the wordplay.
'scoundrel' becomes 'cur' (cur can mean a despicable person).
'opposed to' becomes 'v' (abbreviation for versus).
'ecstasy' becomes 'E' (informal term for the drug).
'cur'+'v'+'e'='CURVE'
(Other definitions for curve that I've seen before include "Comparable in some respects" , "small bulge" , "way of learning" , "Gentle bend, arc" , "No part of this line is straight" .)