Versatile building block of life (4,4)
I believe the answer is:
stem cell
'versatile building block of life' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for stem cell that I've seen before include "Generalised producer in living organism" , "Undifferentiated functional unit in an organism" , "biological germ" , "Undifferentiated unit of life" , "body-builder" .)