A small seal or baby swan, we hear (6)
I believe the answer is:
signet
'a small seal' is the definition.
(type of official seal)
'baby swan we hear' is the wordplay.
'baby swan' becomes 'cygnet' (I've seen this in another clue).
'we hear' indicates a 'sounds like' (homophone) clue.
'cygnet' is a homophone of 'SIGNET'.
'or' acts as a 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" .)