We are ensconced in the Hill Building (5)
I believe the answer is:
tower
'building' is the definition.
(I've seen this before)
'we are ensconced in the hill' is the wordplay.
'are ensconced in' is an insertion indicator.
'the hill' becomes 'tor' (tor is a kind of hill).
'we' placed inside 'tor' is 'TOWER'.
(Other definitions for tower that I've seen before include "Tall column" , "Reach high" , "famous prison" , "Skyscraper" , "Steeple" .)