Sound of seal or young swan (6)
I believe the answer is:
cygnet
'young swan' is the definition.
(a cygnet is a young swan)
'sound of seal' is the wordplay.
'sound of' shows a homophone (sound like).
'seal' becomes 'signet' (signet is a kind of seal).
'signet' sounds like 'CYGNET'.
'or' acts as a link.
(Other definitions for cygnet that I've seen before include "Young aquatic bird" , "Aquatic creature" , "Immature swan" , "Small pen, perhaps?" , "Young water bird" .)