Lose argument in the Baltic, say (3)
I believe the answer is:
sea
'the baltic say' is the definition.
(baltic is a kind of sea)
'lose argument in' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SEA' is hidden in the letters of 'lose argument'.
(Other definitions for sea that I've seen before include "Part of ocean" , "Ocean area" , "one may be pounding the beach" , "Division of an ocean" , "that's easily reached from Dover?" .)