Gripped by sudden fear, topless opponents in game lacking vital element (6)

'lacking vital element' is the definition.
(lacking oxygen)

'gripped by sudden fear topless opponents in game' is the wordplay.
'gripped by' is an insertion indicator.
'sudden fear' becomes 'panic' (I've seen this before).
'topless' means to remove the first letter.
'opponents in game' becomes 'ox' (as in the game noughts and crosses).
'panic' with its initial letter taken off is 'anic'.
'anic' enclosing 'ox' is 'ANOXIC'.