Unit of heredity (4)
I believe the answer is:
gene
'unit of heredity' is the definition.
(heredity means passing on genetic characteristics)
This is the entire clue.
(Other definitions for gene that I've seen before include "Something got from parent < for boy" , "The Selfish -- (Dawkins)" , "Biological factor determining heredity" , "DNA unit" , "Hereditary determinant" .)