Subject mixing hype with aggro (9)
I believe the answer is:
geography
'subject' is the definition.
(geography is a kind of subject)
'mixing hype with aggro' is the wordplay.
'mixing' indicates an anagram.
'with' says to put letters next to each other.
'hype' after 'aggro' is 'aggrohype'.
'aggrohype' with letters rearranged gives 'GEOGRAPHY'.
(Other definitions for geography that I've seen before include "World study" , "Study of earth's topography and man's responses to it" , "Science of the surface of the earth and its inhabitants" , "The study of the earth" , "Scientific study of earth's surface" .)