Seal, or young bird, do we hear? (6)
I believe the answer is:
signet
'seal' is the definition.
(I know that signet is a type of seal)
'young bird do we hear?' is the wordplay.
'young bird' becomes 'cygnet' (young swan).
'do 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 - type of ring" , "Small seal, on ring perhaps" , "'Seal, on a ring say (6)'" , "Small seal set in a ring" .)