Naval craft to change direction for officer (9)
I believe the answer is:
subaltern
'for officer' is the definition.
(subaltern is a kind of officer)
'naval craft to change direction' is the wordplay.
'naval craft' becomes 'sub' (submarine).
'to change' becomes 'alter' (synonyms).
'direction' becomes 'N' (N is an example).
'sub'+'alter'+'n'='SUBALTERN'
(Other definitions for subaltern that I've seen before include "Lesser official" , "Army officer below the rank of captain" , "Junior officer" .)