Kelly or Autry (4)
I believe the answer is:
gene
This clue is a double definition.
'kelly' is the first definition.
(I've seen this in other clues)
'autry' is the second definition.
(I've seen this in another clue)
(Other definitions for gene that I've seen before include "Element of heredity" , "Unit of heredity significance" , "Biological factor determining heredity" , "affects someone's make-up" , "Basic unit of heredity" .)