Small organisms, hundred on piece of wood (8)
I believe the answer is:
plankton
'small organisms' is the definition.
(small sea-dwelling organisms)
'hundred on piece of wood' is the wordplay.
'hundred' becomes 'ton' (I've seen this before).
'on' means one lot of letters go next to another.
'piece of wood' becomes 'plank'.
'ton' put after 'plank' is 'PLANKTON'.
(Other definitions for plankton that I've seen before include "Simple marine life" , "Tiny floating organisms" , "Drifting marine organisms" , "Microscopic organic life that floats in sea" , "Drifting ocean organisms" .)