Sailor's call (4)
I believe the answer is:
ahoy
'sailor's call' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ahoy that I've seen before include "Sailor's shout to ship" , "Shout from sailor" , "Nautical shout" , "Call for ship's attention" , "Nautical call for attention" .)