Food for school, possibly great weight on board (8)
I believe the answer is:
plankton
'food for school possibly great' is the definition.
I can't judge whether this definition defines the answer.
'weight on board' is the wordplay.
'weight' becomes 'ton' (unit of weight).
'on' is a charade indicator (letters next to each other).
'board' becomes 'plank' (synonyms).
'ton' after 'plank' is 'PLANKTON'.
Can you help me to learn more?
(Other definitions for plankton that I've seen before include "Aggregate of small marine organisms" , "Drifters in the sea" , "Sea-food eaten by whales" , "Tiny organisms drifting in the sea" , "Minute fish food" .)