Unit of inheritance (4)
I believe the answer is:
gene
'unit of inheritance' 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 "Biological factor determining heredity" , "Unit of heredity significance" , "The Selfish -- (Dawkins)" , "___ Kelly, star of Singin' In The Rain, d. 1996" , "and 16 Down: Rockabilly singer" .)