Micro-organisms that can cause disease (8)
I believe the answer is:
bacteria
'micro-organisms that can cause disease' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bacteria that I've seen before include "Causes of many diseases" , "Infectious organisms" , "Germs or bugs" , "Single-celled microorganisms" , "Possible cause of disease" .)