Baltic and northern wader (7)
I believe the answer is:
bittern
'wader' is the definition.
(wading bird)
'baltic and northern' is the wordplay.
I cannot quite see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
The remaining letters 'bitter' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bittern that I've seen before include "found hidden in reed-beds" , "Marsh bird of the heron family" , "Aquatic bird" , "Bird of marshes" , "inhabitant of marsh" .)