One thought he was ugly until he started swanning around (6)
I believe the answer is:
cygnet
'ugly until he started swanning around' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'one thought he' is the wordplay.
I cannot quite see how this works, but
'he' could be 'cy' and 'cy' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'was' is the link.
Can you help me to learn more?
(Other definitions for cygnet that I've seen before include "Young aquatic bird" , "Aquatic creature" , "small pen? Possibly" , "Youthful swan" , "Young bird of a particular type" .)