Nautical hail (4)
I believe the answer is:
ahoy
'nautical hail' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ahoy that I've seen before include "Seafarer's call" , "Mariner's challenge" , "Call for ship's attention" , "Nautical shout" , "Hullo, sailor!" .)