Keep mum, say, closer (7)

I believe the answer is:

shutter

'closer' is the definition.

'keep mum say' is the wordplay.
'keep mum' becomes 'sh' (both are ways of telling someone to be quiet).
'say' becomes 'utter' (I've seen this before).
'sh'+'utter'='SHUTTER'

(Other definitions for shutter that I've seen before include "Hinged window cover" , "Camera mechanism" , "application for Windows?" , "It helps provide shade" , "Removable cover" .)

I've seen this clue in The Times.

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