Window’s old — top requires replacement (6)

I believe the answer is:

dormer

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'window's' is the definition.
(dormer is a kind of window)

I don't understand the remainder of the clue.

(Other definitions for dormer that I've seen before include "Type of window set in sloping roof" , "Window projecting vertically from a sloping roof" , "Window fitted vertically onto a sloping roof." , "Light" , "Upright window built out from a sloping roof" .)

I've seen this clue in The Hindu.

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