Idiot put on weight: that's seafood (8)
I believe the answer is:
plankton
'that's seafood' is the definition.
(I've seen this before)
'idiot put on weight' is the wordplay.
'idiot put' becomes 'plank' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' says to put letters next to each other.
'weight' becomes 'ton' (unit of weight).
'plank'+'ton'='PLANKTON'
Can you help me to learn more?
(Other definitions for plankton that I've seen before include "drifting masses" , "Whale food" , "Drifters in the sea" , "Tiny creatures" , "Drifting aquatic organisms" .)