Small seal picked up young swimmer (6)
I believe the answer is:
cygnet
'young swimmer' is the definition.
(a young swan)
'small seal picked up' is the wordplay.
'small seal' becomes 'signet' (I've seen this before).
'picked up' indicates a 'sounds like' (homophone) clue.
'signet' sounds like 'CYGNET'.
(Other definitions for cygnet that I've seen before include "Young aquatic bird" , "Baby swan (ugly duckling?)" , "one seen swimming near pen" , "Young water-bird" , "Little swan" .)