Treatise on a part of the country (5)
I believe the answer is:
tract
This clue might be a double definition.
'treatise' is the first definition.
(tract is a kind of treatise)
'part of the country' is the second definition.
'tract' can be an answer for 'part' (tract is a kind of part). I am not certain of the 'of the country' bit.
'on a' is the link.
Can you help me to learn more?
(Other definitions for tract that I've seen before include "Pamphlet, large expanse of land" , "Extended area of land" , "Region; pamphlet" , "religious work in > area" , "anthem" .)