Sea of northern Europe (6)
I believe the answer is:
baltic
'sea of northern europe' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for baltic that I've seen before include "Nearly land-locked sea of Scandinavia" , "cold waters" , "Almost land-locked sea of NE Europe" , "North European sea" , "Near-landlocked sea of N Europe" .)