Atlantic arm (6)
'atlantic arm' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for baltic that I've seen before include "cold waters" , "Scandinavian sea" , "Northern European sea" , "Sea of northern Europe" , "Coming from Tallinn, perhaps" .)
(Other definitions for irish sea that I've seen before include "Water west of Wales" , "Local arm of Atlantic" , "wet patch" , "Water to west of England" , "It separates Ireland from Britain" .)