Treatise on a part of the country (5)
I believe the answer is:
tract
'treatise' is the definition.
(tract is a kind of treatise)
'on a part of the country' is the wordplay.
'on' says to put letters next to each other.
'a part of' says to take the initial letters (I've seen 'part' mean this).
'country' becomes 'trac' (I can't explain this - if you can you should believe this answer much more).
The initial letter of 'the' is 't'.
't' after 'trac' is 'TRACT'.
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" .)