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