Maintain part of castle (4)
I believe the answer is:
This clue is a double definition.
'maintain' is the first definition.
('keep' can be a synonym of 'maintain')
'part of castle' is the second definition.
('keep' is part of 'castle')
(Other definitions for keep that I've seen before include "Retain; strong tower" , "Central tower of castle" , "Conserve" , "Stock" , "Living" .)