Extremely cold, close to fen, for this marsh bird? (7)
I believe the answer is:
bittern
'this marsh bird?' is the definition.
(I've seen this before)
'extremely cold close to fen' is the wordplay.
'extremely cold' becomes 'bitter' (bitterly cold).
'close to' suggests the final letters (the close of something is the end).
The final letter of 'fen' is 'n'.
'bitter'+'n'='BITTERN'
'for' acts as a link.
(Other definitions for bittern that I've seen before include "Bird with a booming cry" , "'He shall not hear the . . . cry' (Ledwidge)" , "''He shall not hear the ... cry / In the wild sky where he is lain'' (Lament for Thomas MacDonagh)" , "flier" , "inhabitant of marsh" .)