Hail, at sea, a boat (4)
I believe the answer is:
ahoy
'hail at sea' is the definition.
(I've seen this before)
'a boat' is the wordplay.
'boat' becomes 'hoy' (hoy is a kind of boat**).
'a'+'hoy'='AHOY'
(Other definitions for ahoy that I've seen before include "Hello there on board!" , "Call for attention at sea" , "Mariner's challenge" , "Sailor's greeting" , "Shout from sailor" .)