Marine ____ (5)
'marine' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for nautical that I've seen before include "Marine" , "Relating to sailors" , "Seafaring" , "about boats" , "To do with ships and the sea" .)
(Other definitions for corps that I've seen before include "Army subdivision" , "Specialised mil. body" , "Military body" , "Army division" , "fighting group" .)