The key to a high state of readiness? (7,5)

I believe the answer is:

concert pitch

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'state of readiness?' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for concert pitch that I've seen before include "state of readiness" , "A universally-agreed standard" , "state of alertness" .)

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

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