DNA unit (4)
I believe the answer is:
gene
'dna unit' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gene that I've seen before include "affects someone's make-up" , "Element of heredity" , "___ Kelly, star of Singin' In The Rain, d. 1996" , "Hereditary determinant" , "Bodily factor controlling heredity" .)