Retain part of the castle (4)
I believe the answer is:
keep
I believe this clue is a triple definition!
'retain' is the first definition.
(I know that retain 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.
'the castle' is the third definition.
(keep is a type of castle)
(Other definitions for keep that I've seen before include "honour" , "Dungeon" , "Main tower within the walls of a castle" , "Retain; part of castle" , "Subsistence" .)