Old salt (3,3)
'old salt' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sea dog that I've seen before include "Ancient Mariner" , "Old tar" , "Experienced mariner" , "'Old salt (3,3)'" , "mythical creature" .)
(Other definitions for seadog that I've seen before include "Mate, perhaps" , "Rating" .)