Microscopic creature (6)
I believe the answer is:
amoeba
'microscopic creature' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for amoeba that I've seen before include "Unicellular form" , "little creature" , "Protozoan of ever-changing shape" , "Microscopic organism of single cell that changes shape constantly" , "Protozoon" .)