A bunion, say, won't help a tennis player (4,5)

I believe the answer is:

foot fault

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'player' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for foot fault that I've seen before include "Bad service" , "twenty-six [ACHILLES HEEL] could be so described" .)

I've seen this clue in the Evening Standard.

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