Microscopic life form
I believe the answer is:
amoeba
'microscopic life form' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for amoeba that I've seen before include "Tiny one-celled animal in water" , "Tiny organism consisting of single changing cell" , "Single-celled animal" , "Freshwater or marine protozoa" , "Microscopic life form" .)