Hard material covering a small window (8)

I believe the answer is:

casement

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'window' is the definition.
(casement is a kind of window)

'hard material covering a small' is the wordplay.
'hard material' becomes 'cement' (cement is a kind of material. I am not sure about the 'hard' bit.).
'covering' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'='as'
'cement' going around 'as' is 'CASEMENT'.

Can you help me to learn more?

(Other definitions for casement that I've seen before include "Sir Roger" , "Window opening like a door" , "Irish nationalist" , "Side-opening window" , "Hinged window sash" .)

I've seen this clue in The Times.

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