Microscopic organisms in water (8)
I believe the answer is:
plankton
'microscopic organisms in water' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for plankton that I've seen before include "Forms of microscopic life that float in sea" , "Drifting ocean organisms" , "drifting masses" , "Diminutive drifter" , "Drifting marine organisms" .)