Part of the "back forty" (4)
I believe the answer is:
acre
'part of the back forty' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acre that I've seen before include "Care about this bit of land" , "Care about the piece of land" , "0.405 of a hectare" , "Muslims here besieged" , "Mediterranean port" .)