Part of a pool (4)
I believe the answer is:
gene
'part of a pool' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gene that I've seen before include "Bodily factor controlling heredity" , "Hereditary unit on chromosome" , "Basic unit of heredity" , "A segment of DNA" , "Hereditary DNA sequence" .)