Part of a chromosome (4)
I believe the answer is:
gene
'part of a chromosome' is the definition.
('gene' is part of 'chromosome')
This is the entire clue.
(Other definitions for gene that I've seen before include "affects someone's make-up" , "What makes us different" , "and 16 Down: Rockabilly singer" , "-- Pitney, singer" , "- - Kelly, dancer" .)