Check place one's locked up potent biological unit (4,4)
I believe the answer is:
stem cell
'potent biological unit' is the definition.
(type of cell)
'check place one's locked up' is the wordplay.
'check' becomes 'stem' (both can mean to control or stop).
'place one's locked up' becomes 'cell' (as in a prison cell).
'stem'+'cell'='STEM CELL'
(Other definitions for stem cell that I've seen before include "Undifferentiated unit of life" , "genetic unit" , "biological germ" , "Undifferentiated functional unit in an organism" , "Generalised producer in living organism" .)