Population theorist originally had useful stats on cereal (7)
I believe the answer is:
malthus
'population theorist' is the definition.
(Thomas Robert Malthus)
'originally had useful stats on cereal' is the wordplay.
'originally' suggests taking the first letters.
'on' is a charade indicator (letters next to each other).
'cereal' becomes 'malt' (malt is a kind of cereal**).
The initial letters of 'had useful stats' is 'hus'.
'hus' put after 'malt' is 'MALTHUS'.
(Other definitions for malthus that I've seen before include "economist" , "Prophet of overpopulation, d.1834" .)