One not returning into rough sea somewhere on the Baltic (7)
I believe the answer is:
estonia
'somewhere on the baltic' is the definition.
'one not returning into rough sea' is the wordplay.
'one' becomes 'i' (Roman numeral).
'returning' says the letters should be written backwards.
'into' indicates putting letters inside.
'rough' indicates an anagram.
'i'+'not'='inot'
'inot' backwards is 'toni'.
'sea' with letters rearranged gives 'esa'.
'toni' placed into 'esa' is 'ESTONIA'.
(Other definitions for estonia that I've seen before include "Country on the Baltic Sea" , "So eat in here in Eastern Europe" , "Baltic state" , "Land" , "East European country" .)