Lose argument about 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' is hidden within 'lose argument'.
(Other definitions for sea that I've seen before include "E.g. Baltic" , "7 [ship] s on this" , "70% of the earth's surface" , "The tide" , "All at . . . - totally confused" .)