Small seal sounds like small swan (6)
I believe the answer is:
signet
'small seal' is the definition.
(I've seen this before)
'sounds like small swan' is the wordplay.
'sounds like' indicates a 'sounds like' (homophone) clue.
'small swan' becomes 'cygnet' (A cygnet is a young swan).
'cygnet' sounds like 'SIGNET'.
(Other definitions for signet that I've seen before include "Tuft of hanging threads" , "Small seal, on ring perhaps" , "Little seal" , "'Seal, on a ring say (6)'" , "Small seal - type of ring" .)