Sound of baby bird or seal (6)
I believe the answer is:
signet
'seal' is the definition.
(I know that signet is a type of seal)
'sound of baby bird' is the wordplay.
'sound of' shows a homophone (sound like).
'baby bird' becomes 'cygnet' (a cygnet is a young swan).
'cygnet' is a homophone of 'SIGNET'.
'or' acts as a link.
(Other definitions for signet that I've seen before include "Small seal sounds like a young swan" , "Tuft of hanging threads" , "Small seal, on ring perhaps" , "Small seal - type of ring" , "Ingest in ring" .)