Small seal sounding like a young swan! (6)
I believe the answer is:
signet
'small seal' is the definition.
(type of official seal)
'sounding like a young swan' is the wordplay.
'sounding like' indicates a 'sounds like' (homophone) clue.
'a young swan' becomes 'cygnet' (a cygnet is a young swan).
'cygnet' is a homophone of 'SIGNET'.
(Other definitions for signet that I've seen before include "Small seal set in a ring" , "Tinges (anag) - seal" , "Kind of ring with seal" , "Tuft of hanging threads" , "Small seal sounds like a young swan" .)