Young swan (6)
I believe the answer is:
cygnet
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">young swan</span>' is the definition.<br/ ><span class="explanation-format__explanation">(a cygnet is a young swan)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for cygnet that I've seen before include "Little swan" , "Young swan" , "that's no ugly duckling" , "one seen swimming near pen" , "Immature swan" .)