A small seal or baby swan, we hear (6)
I believe the answer is:
signet
'a small seal' is the definition.
(I've seen this before)
'baby swan we hear' is the wordplay.
'baby swan' becomes 'cygnet' (I've seen this before).
'we hear' indicates a 'sounds like' (homophone) clue.
'cygnet' is a homophone of 'SIGNET'.
'or' is the link.
(Other definitions for signet that I've seen before include "Tuft of hanging threads" , "Small seal, on ring perhaps" , "Small seal set in a ring" , "Little seal" , "Official seal" .)