Turning over headwear before measuring (9)

I believe the answer is:

capsizing

'turning over' is the definition.
(capsizing is a kind of turning over)

'headwear before measuring' is the wordplay.
'headwear' becomes 'cap' (I've seen this before).
'before' says to put letters next to each other.
'measuring' becomes 'sizing' (I've seen this before).
'cap'+'sizing'='CAPSIZING'

(Another definition for capsizing that I've seen is " Turning turtle".)

I've seen this clue in The Telegraph.

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