Screen in position for start of show (7,2)

I believe the answer is:

curtain up

'start of show' is the definition.

'screen in position' is the wordplay.
'screen' becomes 'curtain' (curtain is a kind of screen).
'in position' becomes 'up' (to be up is to be in a particular position).
'curtain'+'up'='CURTAIN-UP'

'for' acts as a link.

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

I've seen this clue in The Guardian.

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