Farm acre in the middle of Dorset, at last (5)
I believe the answer is:
croft
'farm' is the definition.
(I know that croft is a type of farm)
'acre in the middle of dorset at last' is the wordplay.
'in the middle' means to look at the middle letters.
'at last' says to take the final letters.
The middle letters of 'acre' are 'cr'.
The final letter of 'dorset' is 't'.
'cr'+'of'+'t'='CROFT'
(Other definitions for croft that I've seen before include "Small Scottish rented farm" , "Virgin Mary's husband" , "Scottish smallholding" , "Plot" , "Scottish farmstead" .)