Very cold November for a marsh bird (7)
I believe the answer is:
bittern
'a marsh bird' is the definition.
(I've seen this before)
'very cold november' is the wordplay.
'very cold' becomes 'bitter' (bitter informally can mean very cold).
'november' becomes 'n' (phonetic alphabet: alpha, bravo, charlie etc.).
'bitter'+'n'='BITTERN'
'for' acts as a link.
(Other definitions for bittern that I've seen before include "Bird that booms" , "Marsh bird of the heron family" , "inhabitant of marsh" , "''He shall not hear the ... cry / In the wild sky where he is lain'' (Lament for Thomas MacDonagh)" , "Bird of marshes" .)