Maintain part of castle (4)
I believe the answer is:
keep
This clue is a triple definition!
'maintain' is the first definition.
(synonyms)
'part' is the second definition.
(I have seen 'part of castle' mean 'keep' so perhaps 'part' could also mean 'keep')
'of' is 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" .)