Set of stairs incorporating an overhead window (8)
I believe the answer is:
Here is my best explanation:
'overhead window' is the definition.
'set of stairs incorporating an' is the subsidiary indication.
'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 "elevated aperture", "Thin flag (anag.)", "Rectangular window over a door", "It enables us to see", "kind of window", "Window over door", "Windows above a door", "upper window?", "Small window" and "opening over the door".)