Greeting at sea (4)
I believe the answer is:
ahoy
'greeting at sea' 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 "Call for ship's attention" , "Hullo, sailor!" , "Sailor's shout for attention" , "Seafarer's call" , "What a sailor may" .)