Heredity unit (4)
I believe the answer is:
gene
'heredity unit' 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 "- - Kelly, dancer" , "Bodily factor controlling heredity" , "___ Kelly, star of Singin' In The Rain, d. 1996" , "may be passed to child" , "Biological factor determining heredity" .)