Staircase containing a new window (8)
I believe the answer is:
fanlight
'window' is the definition.
(fanlight is a kind of window)
'staircase containing a new' is the wordplay.
'staircase' becomes 'flight' (flight is a kind of staircase).
'containing' indicates putting letters inside.
'a new' becomes 'an' (I've seen this in other clues).
'flight' placed around 'an' is 'FANLIGHT'.
(Other definitions for fanlight that I've seen before include "Windows above a door" , "opening over the door" , "Rectangular window over a door" , "upper window?" , "elevated aperture" .)