Bet on a horse that's out of commission (4,2)

I believe the answer is:

laid up

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'out of commission' is the definition.
Both the definition and answer are adjectives. Maybe there's a link between them I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for laid up that I've seen before include "in sickbed" , "Dicky" , "Injured" , "Bedridden" , "Stored, preserved" .)

I've seen this clue in The Independent.

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