Marchers drop their odd spot cards (4)

I believe the answer is:

aces

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'cards' is the definition.
(ace is a kind of card)

I can't explain the rest of the clue.

(Other definitions for aces that I've seen before include "excellent service" , "Winning serves" , "Honours" , "Ones or elevens in cards" , "Experts (in their field)" .)

I've seen this clue in the Sydney Morning Herald.

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