Facts on English hereditary unit (4)
I believe the answer is:
gene
'hereditary unit' is the definition.
(I've seen this in another clue)
'facts on english' is the wordplay.
'facts' becomes 'gen'.
'on' means one lot of letters go next to another.
'english' becomes 'e' (abbreviation).
'gen'+'e'='GENE'
(Other definitions for gene that I've seen before include "and 16 Down: Rockabilly singer" , "Something got from parent < for boy" , "A segment of DNA" , "Basic unit of heredity" , "DNA unit" .)