Call to a mate (4)
'call to a mate' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ahoy that I've seen before include "Sailor's shout to ship" , "Nautical greeting" , "Hello sailor!" , "Call for ship's attention" , "Nautical call for attention" .)
(Other definitions for gday that I've seen before include "greeting from Adelaide?" , "welcome in Australia" , "Aussie opener" .)