Small window 75 per cent dark? (12)

I believe the answer is:

quarterlight

I'm a little stuck... Click here to teach me more about this clue!

'small window' is the definition.
'quarterlight' can be an answer for 'window' (quarterlight is a kind of window). I am not sure about the 'small' bit.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for quarterlight that I've seen is " Small, pivoted window in car".)

I've seen this clue in The Times.

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