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 a hidden word (the clue goes around or about the answer).
'SEA' can be found hidden inside 'lose argument'.
'the' acts as a 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" .)