Left gap for window (8)
I believe the answer is:
porthole
'for window' is the definition.
(porthole is a kind of window)
'left gap' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'gap' becomes 'hole' (hole is a kind of gap).
'port'+'hole'='PORTHOLE'
(Other definitions for porthole that I've seen before include "Small watertight opening in ship's side" , "Circular window in ship" , "Through which sailor can see" , "from which sea can be seen" , "firing through this" .)