Zero pressure working in the midst of confined enemy (8)
I believe the answer is:
'enemy' is the definition.
'zero pressure working in the midst of confined' is the wordplay.
'zero' becomes 'o'.
'pressure' becomes 'p' (abbreviation).
'working' becomes 'on'.
'in the midst of' means one lot of letters goes inside another.
'confined' becomes 'pent' (similar in meaning).
'on' placed inside 'pent' is 'ponent'.
(Other definitions for opponent that I've seen before include "Contestant you're matched against", "Challenger", "One's competitor", "Foe or competitor", "member of other team".)