Main entrance to top stately home (4)
I believe the answer is:
seat
'stately home' is the definition.
'main entrance to top' is the wordplay.
'main' becomes 'sea' ('sea' can be a synonym of 'main').
'entrance to' says to take the initial letters.
The initial letter of 'top' is 't'.
'sea'+'t'='SEAT'
(Other definitions for seat that I've seen before include "Settle in" , "A manner of sitting on horse" , "Spanish car" , "Parliamentary constituency" , "MP's place in parliament" .)