Castle guard (4)
I believe the answer is:
keep
This clue is a double definition.
'castle' is the first definition.
(thesaurus)
'guard' is the second definition.
(keeping guard is a kind of guarding)
(Other definitions for keep that I've seen before include "Main tower of a medieval castle" , "strong position" , "Retain; strong tower" , "run" , "Living" .)