A sneak initially enters secure window (8)
I believe the answer is:
casement
'window' is the definition.
(thesaurus)
'a sneak initially enters secure' is the wordplay.
'initially' suggests taking the first letters.
'enters' is an insertion indicator.
'secure' becomes 'cement' (cementing is a kind of securing**).
The initial letter of 'sneak' is 's'.
'a'+'s'='as'
'as' going inside 'cement' is 'CASEMENT'.
(Other definitions for casement that I've seen before include "Vertical-hinged window" , "Sir Roger" , "Hinged window-sash" , "Window for the patriot?" , "Window opening on a vertical hinge" .)