Micro-organisms (8)
I believe the answer is:
bacteria
'micro-organisms' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bacteria that I've seen before include "Tiny causes of disease" , "Microscopic organisms, can carry disease" , "Single-celled organisms" , "Single-cell organisims" , "Single-cell organisms, can be good or bad for you" .)