Idiot has not served up seafood (8)
I believe the answer is:
plankton
'seafood' is the definition.
(I've seen this before)
'idiot has not served up' is the wordplay.
'idiot has' becomes 'plank' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'served up' says the letters should be written backwards (in a down clue, going upwards).
'not' backwards is 'ton'.
'plank'+'ton'='PLANKTON'
Can you help me to learn more?
(Other definitions for plankton that I've seen before include "food for school, possibly" , "Whale's staple diet" , "Aggregate of small marine organisms" , "Marine organism" , "Drifting marine organisms" .)