Revolutionary nature film’s set in Middle-earth (7)
I believe the answer is:
tropics
'middle-earth' is the definition.
(the tropics are in the middle of the planet)
'revolutionary nature film's set in' is the wordplay.
'revolutionary' says the letters should be written backwards.
'nature' becomes 'sort' (both can mean a variety or kind).
'film' becomes 'pic' (short for picture).
'set in' means one lot of letters goes inside another.
'sort' backwards is 'tros'.
'tros' enclosing 'pic' is 'TROPICS'.
(Other definitions for tropics that I've seen before include "heated situations" , "torrid zones" , "Cancer, Capricorn" , "hot spot" , "hot regions" .)