Very cold northern bird (7)
I believe the answer is:
bittern
'bird' is the definition.
(bittern is a kind of bird)
'very cold northern' is the wordplay.
'very cold' becomes 'bitter' (bitter informally can mean very cold).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'bitter'+'n'='BITTERN'
(Other definitions for bittern that I've seen before include "large wader" , "inhabitant of marsh" , "found hidden in reed-beds" , "Aquatic bird" , "''He shall not hear the ... cry / In the wild sky where he is lain'' (Lament for Thomas MacDonagh)" .)