Crossword Genius

Maybe marathon runner’s standard habitual reaction: a single short breath (11)

Ross

I believe the answer is:

participant

I'm a little stuck... Click here to teach me more about this clue!

'short breath' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'maybe marathon runner's standard habitual reaction a single' is the wordplay.
I cannot quite understand how this works, but
'standard' could be 'par' (standard of play in golf) and 'par' is located in the answer.
'a' could be 'an' and 'an' is located in the answer.
'single' could be 'i' (one) and 'i' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for participant that I've seen before include "One getting involved in something" , "Competitor" , "Actor" , "player" , "One actively involved" .)

I've seen this clue in The Times.
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