Seal audibly was a youngster (6)
I believe the answer is:
cygnet
'a youngster' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'seal audibly' is the wordplay.
'seal' becomes 'signet' (signet is a kind of seal).
'audibly' shows a homophone (sound like).
'signet' is a homophone of 'CYGNET'.
'was' acts as a link.
Can you help me to learn more?
(Other definitions for cygnet that I've seen before include "Young water bird" , "Sounds like the type of ring" , "Immature swan" , "Young aquatic bird" , "Little swan" .)