Young bird making noise like a small seal (6)
I believe the answer is:
cygnet
'young bird' is the definition.
(young swan)
'making noise like a small seal' is the wordplay.
'making noise like' indicates a 'sounds like' (homophone) clue.
'a small seal' becomes 'signet' (type of official seal).
'signet' is a homophone of 'CYGNET'.
(Other definitions for cygnet that I've seen before include "one seen swimming near pen" , "Young water-bird" , "Young swan" , "Young aquatic bird" , "Young bird of a particular type" .)