Window when set in mortar (8)
I believe the answer is:
casement
'window' is the definition.
(thesaurus)
'when set in mortar' is the wordplay.
'when' becomes 'as'.
'set in' is an insertion indicator.
'mortar' becomes 'cement' (I've seen this in other clues).
'as' put within 'cement' is 'CASEMENT'.
(Other definitions for casement that I've seen before include "Window that opens with hinges on side, like a door" , "Side-opening window" , "Type of window" , "a man deemed to have betrayed England" , "Window opening like a door" .)