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