Experienced sailor (6)
(Other definitions for seadog that I've seen before include "Mate, perhaps" , "Rating" .)
'experienced sailor' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sea dog that I've seen before include "'Old salt (3,3)'" , "Old or experienced sailor" , "Dosage (anag)" , "Experienced sailor (colloq.)" , "mythical creature" .)