Guide soldiers to castle (6)
I believe the answer is:
mentor
'guide' is the definition.
(I've seen this in another clue)
'soldiers to castle' is the wordplay.
'soldiers' becomes 'men' (men can mean a group of soldiers).
'castle' becomes 'r' (abbreviation for rook/castle in chess).
'men'+'to'+'r'='MENTOR'
(Other definitions for mentor that I've seen before include "A wise and experienced teacher" , "Wise and trusted guide and advisor" , "One who steers" , "Guru" , "Wise counsellor (of protege)" .)