Lose argument about the Baltic, say (3)
I believe the answer is:
sea
'baltic say' is the definition.
(baltic is a kind of sea)
'lose argument about' is the wordplay.
'about' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'SEA' can be found hidden inside 'lose argument'.
'the' is the link.
(Other definitions for sea that I've seen before include "Expanse of salt water" , "one may be pounding the beach" , "North for one" , "The Med, e.g." , "Great expanse of water" .)