Crossword Genius

Stagnation shocked opponent (10)

Ross

I believe the answer is:

antagonist

'opponent' is the definition.
('antagonist' can be a synonym of 'opponent')

'stagnation shocked' is the wordplay.
'shocked' indicates anagramming the letters.
'stagnation' with letters rearranged gives 'ANTAGONIST'.

(Other definitions for antagonist that I've seen before include "Hostile person" , "Adversary" , "Opponent" , "rival" , "5 down (anag)" .)

I've seen this clue in The Times.
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