Preserve part of a castle (4)
I believe the answer is:
This clue is a double definition.
'preserve' is the first definition.
('keep' can be a synonym of 'preserve')
'part of a castle' is the second definition.
('keep' is part of 'castle')
(Other definitions for keep that I've seen before include "Act as custodian of" , "Board and lodging" , "Retain; strong tower" , "Continue" , "Guard" .)