One must be quick to do so (7)

I believe the answer is:

breathe

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'one must be' is the definition.
'breathe' can be an answer for 'be' (thesaurus). I'm unsure of the 'one must' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for breathe that I've seen before include "Expel gas" , "Respire - inspire - expire" , "Take into mouth" , "wheeze" , "Tell" .)

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

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