Start to play (7,2)

I believe the answer is:

curtain up

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'start' is the definition.
(I have seen 'start of play' mean 'curtain up' so perhaps 'start' could also mean 'curtain up')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for curtain up that I've seen is " start of play".)

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

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