— Sea, body of water connected to the North Sea by the Skaggerak, Kattegat and Oresund (6)
I believe the answer is:
baltic
(Other definitions for baltic that I've seen before include "Almost land-locked sea of NE Europe" , "Coming from Tallinn, perhaps" , "The Sea around Scandinavia" , "Near-landlocked sea of N Europe" , "water, lots" .)