Raw recruit that is seen by castle (6)
I believe the answer is:
rookie
'raw recruit' is the definition.
(I've seen this in another clue)
'that is seen by castle' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'seen by' says to put letters next to each other.
'castle' becomes 'rook' (synonyms).
'ie' after 'rook' is 'ROOKIE'.
(Other definitions for rookie that I've seen before include "Novice" , "New (police) recruit (colloq.)" , "new team member" , "untested type" , "inexperienced fighter" .)