Large area in a treatise (5)
I believe the answer is:
tract
This clue could be a double definition.
'large area' is the first definition.
'tract' can be an answer for 'area' (tract is a kind of area). I am not certain of the 'large' bit.
'treatise' is the second definition.
(I know that tract is a type of treatise)
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for tract that I've seen before include "Religious treatise" , "Short treatise; land area" , "Part of body" , "Lots of land" , "Large area of land or small booklet" .)