Adversary of obscene mystery play (5)
I believe the answer is:
enemy
'adversary' is the definition.
(I know that enemy is a type of adversary)
'of obscene mystery play' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ENEMY' is hidden in the letters of 'obscene mystery play'.
(Other definitions for enemy that I've seen before include "Hostile opponent" , "one in opposition" , "Time" , "Antagonist" , "Yemen (anag.)" .)