Actor Hackman (4)
I believe the answer is:
gene
'actor hackman' 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 DNA sequence" , "and 16 Down: Rockabilly singer" , "Embarrassment" , "Hereditary unit on chromosome" , "affects someone's make-up" .)