Young bird and seal mentioned (6)
I believe the answer is:
cygnet
'young bird' is the definition.
(I've seen this before)
'seal mentioned' is the wordplay.
'seal' becomes 'signet' (signet is a kind of seal).
'mentioned' indicates a 'sounds like' (homophone) clue.
'signet' is a homophone of 'CYGNET'.
'and' acts as a link.
(Other definitions for cygnet that I've seen before include "Young aquatic bird" , "Immature swan" , "Little swan" , "Young bird of a particular type" , "Young water-bird" .)