Concrete around a small window (8)
I believe the answer is:
casement
'window' is the definition.
(thesaurus)
'concrete around a small' is the wordplay.
'concrete' becomes 'cement' (cement is a kind of concrete).
'around' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'s'='as'
'cement' going around 'as' is 'CASEMENT'.
(Other definitions for casement that I've seen before include "a man deemed to have betrayed England" , "Window that opens with hinges on side, like a door" , "Sir Roger" , "Roger's window?" , "Window for the patriot?" .)