Mustang, don't peg out now, do you hear? This'll help you breathe! (14)

I believe the answer is:

bronchodilator

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'this'll help you breathe' is the definition.
(drug alleviating asthma)

'mustang don't peg out' is the wordplay.
'mustang' becomes 'broncho' (broncho is a kind of mustang).
'don't peg out' becomes 'dilator' (I can't explain this - if you can you should believe this answer much more).
'broncho'+'dilator'='BRONCHODILATOR'

'now do you hear?' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

I've seen this clue in The Times.

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