Farmer casually comes back, fencing area of land (4)
I believe the answer is:
acre
'area of land' is the definition.
(I've seen this before)
'farmer casually comes back fencing' is the wordplay.
'fencing' says the answer is hidden in the clue (some letters fenced in).
'comes back' is a reversal indicator.
'erca' can be found hidden inside 'farmer casually'.
'erca' written backwards gives 'ACRE'.
(Other definitions for acre that I've seen before include "stretch of land" , "Area measure - Israeli port - Brazilian state" , "Muslims here besieged" , "marine location in Israel" , "Old unit of area - old port" .)