Once delivered it's non-returnable (3)

I believe the answer is:

ace

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'once' is the definition.
Both the definition and answer are adjectives. Maybe there's an association between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ace that I've seen before include "Playing card with single spot" , "Tennis serve that the receiver is unable to reach" , "Test pilot'" , "Top card or winning tennis serve" , "Single unit on playing cards" .)

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

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