You can't play it when breathless (5)
I believe the answer is:
flute
'you' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I don't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for flute that I've seen before include "Narrow wineglass" , "Blown" , "Champagne glass" , "Wine glass; instrument" , "High-pitched woodwind instrument" .)