Small organisms that float in water (8)
I believe the answer is:
plankton
'small organisms that float in water' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for plankton that I've seen before include "Tiny organisms drifting in the sea" , "Drifters in the sea" , "Forms of microscopic life that float in sea" , "Marine organism" , "food for school, possibly" .)