Baltic Sea? No, it turns north (8)
I believe the answer is:
estonian
'baltic' is the definition.
(Estonia is a Baltic country)
'sea? no it turns north' is the wordplay.
'turns' indicates anagramming the letters.
'north' becomes 'N' (synonyms).
'sea'+'no'+'it'='seanoit'
'seanoit' anagrammed gives 'estonia'.
'estonia'+'n'='ESTONIAN'
(Other definitions for estonian that I've seen before include "Baltic state national" , "European language" , "Eastern Europe" , "One from a Baltic state" , "Person from Tallinn, say." .)