Farm plot up in Abercastle (4)
I believe the answer is:
acre
'farm plot' is the definition.
(I've seen this in another clue)
'up in abercastle' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' indicates the answer is hidden within the clue.
'erca' is hidden within 'abercastle'.
'erca' written backwards gives 'ACRE'.
(Other definitions for acre that I've seen before include "Care about this bit of land" , "Unit of land measurement" , "marine location in Israel" , "Persistent tendency" , "Unit of area of 4,840 square yards" .)