Beneficiary seen entering castle gate eagerly (7)
I believe the answer is:
legatee
'beneficiary' is the definition.
(legatee is a kind of beneficiary)
'entering castle gate eagerly' is the wordplay.
'entering' indicates the answer is hidden within the clue.
'LEGATEE' is hidden within 'castle gate eagerly'.
'seen' is the link.
(Other definitions for legatee that I've seen before include "One receiving something left in a will" , "Receiver of money from a will" , "One who gains benefits" , "One receiving a bequest" , "One coming into property" .)