Pen or cob? (4)
I believe the answer is:
swan
This clue is a double definition.
'pen' is the first definition.
(pen is a female swan)
'cob?' is the second definition.
(cob is a kind of swan)
(Other definitions for swan that I've seen before include "Large, web-footed bird" , "Large aquatic bird" , "black, in 7th" , "Shakespeare is called The .... of Avon" , "Whooper or trumpeter?" .)