Unit of heredity (4)
I believe the answer is:
'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" .)