North European sea (6)
I believe the answer is:
baltic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">north european sea</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for baltic that I've seen before include "N European sea" , "The Sea around Scandinavia" , "Nearly land-locked sea of Scandinavia" , "Near-landlocked sea of N Europe" , "Coming from Tallinn, perhaps" .)