Guard part of castle (4)
I believe the answer is:
keep
I believe this clue is a double definition.
'guard' is the first definition.
(keeping guard is a kind of guarding)
'part of castle' is the second definition.
('keep' is part of 'castle')
(Other definitions for keep that I've seen before include "run" , "Main tower of a medieval castle" , "Act as custodian of" , "Look after" , "Dungeon" .)