Cob or pen (4)
I believe the answer is:
swan
I believe this is a double definition.
'cob' is the first definition.
(I've seen this in another clue)
'pen' is the second definition.
(pen is a female swan)
(Other definitions for swan that I've seen before include "Cob - pen" , "Bird of Cygnus genus" , "Kind of song, in the end" , "Whooper or trumpeter?" , "... Australian river" .)