At sea, entered marina leaving sea (13)
I believe the answer is:
mediterranean
'sea' is the definition.
(Mediterranean is an example)
'at sea entered marina' is the wordplay.
'at sea' indicates an anagram.
'entered'+'marina'='enteredmarina'
'enteredmarina' anagrammed gives 'MEDITERRANEAN'.
'leaving' is the link.
(Other definitions for mediterranean that I've seen before include "Sea north of Africa" , "Sea south of Europe" , "Inland sea" , "Sea linking three continents" .)