Bit of personal make-up? (4)
I believe the answer is:
gene
'bit of personal make-up?' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for gene that I've seen before include "Unit of heredity significance" , "Hereditary DNA sequence" , "Bodily factor controlling heredity" , "___ Kelly, star of Singin' In The Rain, d. 1996" , "Hereditary determinant" .)