Mustang possibly heading north through dry region (5)
I believe the answer is:
tract
'region' is the definition.
(both can mean a large piece of land)
'mustang possibly heading north through dry' is the wordplay.
'mustang possibly' becomes 'car' (mustang is a type of car).
'heading north' is a reversal indicator (in a down clue, letters travel north).
'through' indicates putting letters inside (inserted letters go through the word).
'dry' becomes 'tt' (abbreviation for teetotal).
'car' back-to-front is 'rac'.
'rac' going inside 'tt' is 'TRACT'.
(Other definitions for tract that I've seen before include "anthem" , "Piece of writing" , "Large area of land or small booklet" , "Political pamphlet" , "religious work in > area" .)