Sound of small seal or young swan (6)
I believe the answer is:
cygnet
'young swan' is the definition.
(a cygnet is a young swan)
'sound of small seal' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'small seal' becomes 'signet' (type of official seal).
'signet' sounds like 'CYGNET'.
'or' is the link.
(Other definitions for cygnet that I've seen before include "Aquatic creature" , "Baby swan (ugly duckling?)" , "Sounds like the type of ring" , "Young swan" , "Youthful swan" .)