Heavy weight on board, whale food (8)
I believe the answer is:
plankton
'whale food' is the definition.
(I've seen this before)
'heavy weight on board' is the wordplay.
'heavy weight' becomes 'ton' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'board' becomes 'plank' ('plank' can be a synonym of 'board').
'ton' put after 'plank' is 'PLANKTON'.
(Other definitions for plankton that I've seen before include "Whale's staple diet" , "Drifters in the sea" , "Sea-food eaten by whales" , "food for school, possibly" , "Whale food" .)