Preserve part of a castle (4)
I believe the answer is:
keep
This clue is a triple definition!
'preserve' is the first definition.
(I know that preserve is a more specific form of the action keep)
'part' is the second definition.
(I have seen 'part of castle' mean 'keep' so perhaps 'part' could also mean 'keep')
'of' acts as a link.
'a castle' is the third definition.
(I've seen this in other clues)
(Other definitions for keep that I've seen before include "Act as custodian of" , "Board and lodging" , "Retain; strong tower" , "Continue" , "Guard" .)