Maintain part of castle (4)
I believe the answer is:
keep
I believe this clue is a triple definition!
'maintain' is the first definition.
(I know that maintain can be written as 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.
'castle' is the third definition.
(keep is a type of castle)
(Other definitions for keep that I've seen before include "Retain; strong tower" , "Central tower of castle" , "Conserve" , "Stock" , "Living" .)