Expert covering right amount of land (4)
I believe the answer is:
acre
'amount of land' is the definition.
(I've seen this in another clue)
'expert covering right' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'covering' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'ace' going around 'r' is 'ACRE'.
(Other definitions for acre that I've seen before include "Four thousand, eight hundred and forty square yards" , "Imperial land measurement" , "region of ground" , "Muslims here besieged" , "Care about this bit of land" .)