Some glass as held by a window frame (4)
I believe the answer is:
sash
'a window frame' is the definition.
(I've seen this before)
'some glass as held' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'SASH' is hidden in the letters of 'glass as held'.
'by' is the link.
(Other definitions for sash that I've seen before include "Band - scarf" , "Popular band in Northern Ireland" , "Window frame - shoulder band" , "Waistband - type of window" , "Strip of cloth - type of window" .)