Water-loving youngster — small seal, reportedly (6)
I believe the answer is:
cygnet
'water-loving youngster' is the definition.
'small seal reportedly' is the wordplay.
'small seal' becomes 'signet' (I've seen this before).
'reportedly' shows a homophone (sound like).
'signet' is a homophone of 'CYGNET'.
(Other definitions for cygnet that I've seen before include "Baby swan (ugly duckling?)" , "small pen? Possibly" , "Young swan" , "that's no ugly duckling" , "one seen swimming near pen" .)