Stay in a castle (4)
I believe the answer is:
keep
I believe this is a double definition.
'stay' is the first definition.
(I know that keep is a more specific form of the action stay)
'castle' is the second definition.
(I've seen this in other clues)
'in a' is the link.
(Other definitions for keep that I've seen before include "Retain; stronghold" , "Retain; strong tower" , "Central tower of castle" , "Main tower of a medieval castle" , "Hold - part of castle" .)