Nautical greeting (4)
I believe the answer is:
ahoy
'nautical greeting' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ahoy that I've seen before include "A sailor's call to a ship" , "Hailing term at sea" , "Call for attention at sea" , "Sailor's shout to ship" , "What a sailor may" .)