Its failed participants may well be out of pocket (9)

I believe the answer is:

billiards

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'out of pocket' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for billiards that I've seen before include "Table game at which Mary Queen of Scots excelled" , "Ball game" , "Indoor sport" , "Game with three balls" , "Game played with balls on table with pockets" .)

I've seen this clue in The Globe and Mail.

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