Hereditary unit (4)
I believe the answer is:
gene
'hereditary unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gene that I've seen before include "Hereditary determinant" , "Factor controlling heredity carried by chromosome" , "What makes us different" , "Something got from parent < for boy" , "Part of chromosome" .)