5 down
Game
played
mostly
by
those
out
of
their
depth
(5,4)
Game played mostly by those out of their depth (5,4)
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The whole clue is the definition (& lit or double definition).
How the clue works
The whole clue works as a cryptic definition — water polo is literally a game played in a pool, where most of the players are out of their depth in deep water at any given moment. There's no separate wordplay here; the entire surface reading does the job with a knowing wink.