Part of window can be a nuisance, we hear (4)
I believe the answer is:
pane
'part of window' is the definition.
(I know that pane is part of window)
'a nuisance we hear' is the wordplay.
'a nuisance' becomes 'pain' (synonyms).
'we hear' shows a homophone (sound like).
'pain' sounds like 'PANE'.
'can be' acts as a link.
(Other definitions for pane that I've seen before include "Piece of glass in window" , "Panel of glass" , "Single sheet of glass in a window" .)