Microorganisms race a bit crazily (8)
I believe the answer is:
bacteria
'microorganisms' is the definition.
(I know this)
'race a bit crazily' is the wordplay.
'crazily' is an anagram indicator.
'race'+'a'+'bit'='raceabit'
'raceabit' anagrammed gives 'BACTERIA'.
(Other definitions for bacteria that I've seen before include "Micro-organisms, often pathogens" , "Tiny organisms, may be good or bad" , "could make you feel bad" , "Infectious microorganisms" , "Care a bit about these micro-organisms" .)