Window’s old — top requires replacement (6)
I believe the answer is:
dormer
'window's' is the definition.
(dormer is a kind of window)
I don't understand the remainder of the clue.
Can you help me to learn more?
(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" .)