Reportedly nothing large eaten by one American sea creature (8)
I believe the answer is:
nautilus
'sea creature' is the definition.
'nautilus' can be an answer for 'creature' (nautilus is a kind of creature). I'm not sure about the 'sea' bit.
'reportedly nothing large eaten by one american' is the wordplay.
'reportedly' shows a homophone (sound like).
'nothing' becomes 'nought' (synonyms).
'large eaten by one' becomes 'il' (I am not sure about this - if you are sure you should believe this answer much more).
'american' becomes 'US' (United States).
'nought' is a homophone of 'naut'.
'naut'+'il'+'us'='NAUTILUS'
Can you help me to learn more?
(Other definitions for nautilus that I've seen before include "Sea creature to be found in the usual tin, unusually" , "Usual tin for a small sea creature" , "Spiral-shelled mollusc" , "Nemo's vessel" , "Sea creature - literary submarine" .)