House at the entrance to grounds of a mansion (5)
I believe the answer is:
lodge
'house at the entrance to grounds of a mansion' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lodge that I've seen before include "residence" , "Cabin for seasonal activity - become embedded" , "Rat messing with this > animal home" , "Deposit for gatekeeper's cottage" , "See 9" .)