Set of stairs incorporating an overhead window (8)
I believe the answer is:
fanlight
'overhead window' is the definition.
'set of stairs incorporating an' is the wordplay.
'set of stairs' becomes 'flight' (I've seen this before).
'incorporating' indicates putting letters inside.
'flight' enclosing 'an' is 'FANLIGHT'.
(Other definitions for fanlight that I've seen before include "It enables us to see" , "Thin flag (anag.)" , "Rectangular window over a door" , "elevated aperture" , "opening over the door" .)