pen or cob (4)
I believe the answer is:
swan
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">pen</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(pen is a female swan)</span><br/ ><br/ >'<span class="explanation-format__original">cob?</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(cob is a kind of swan)</span><br/ ><br/ ></span>
(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?" .)