Young swan and small seal heard (6)
I believe the answer is:
cygnet
'young swan' is the definition.
(a cygnet is a young swan)
'small seal heard' is the wordplay.
'small seal' becomes 'signet' (type of official seal).
'heard' shows a homophone (sound like).
'signet' sounds like 'CYGNET'.
'and' is the link.
(Other definitions for cygnet that I've seen before include "Baby swan (ugly duckling?)" , "Young aquatic bird" , "Sounds like the type of ring" , "Little swan" , "Young water-bird" .)