The worn part of the window? (4)
I believe the answer is:
sash
'the window?' is the definition.
(I've seen this before)
'the worn part' is the wordplay.
'the worn' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'part' becomes 'ash' (I have seen 'Part of forest' mean 'ash' so perhaps 'part' could also mean 'ash').
's'+'ash'='SASH'
'of' is the link.
Can you help me to learn more?
(Other definitions for sash that I've seen before include "Ribbon as a symbol of rank" , "Frame for a windowpane" , "(Uniform) strip; window-frame" , "Piece of cloth round waist or shoulder" , "Broad band, the one me father wore?" .)