Kelly's make-up unit (4)
I believe the answer is:
gene
I believe this is a double definition.
'kelly's' is the first definition.
(I've seen this in other clues)
'make-up unit' is the second definition.
(dna)
(Other definitions for gene that I've seen before include "may be passed to child" , "Something got from parent < for boy" , "Hereditary unit on chromosome" , "affects someone's make-up" , "Bodily factor controlling heredity" .)