Facts on eastern bit of heredity (4)
I believe the answer is:
gene
'bit of heredity' is the definition.
(I've seen this in another clue)
'facts on eastern' is the wordplay.
'facts' becomes 'gen'.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'eastern' becomes 'e'.
'gen'+'e'='GENE'
(Other definitions for gene that I've seen before include "The Selfish -- (Dawkins)" , "Unit of heredity significance" , "Something got from parent < for boy" , "Hereditary molecule" , "Vincent" .)