Wrapped in polythene: mysterious foe (5)
I believe the answer is:
enemy
'foe' is the definition.
(I know that foe can be written as enemy)
'wrapped in polythene mysterious' is the wordplay.
'wrapped in' indicates a hidden word.
'ENEMY' can be found hidden inside 'polythene mysterious'.
(Other definitions for enemy that I've seen before include "one in opposition" , "other side (like 16)" , "Hostile opponent" , "Foe, adversary" , "Division" .)