Expert holding 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 holding right' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'holding' is an insertion indicator.
'right' becomes 'r' (abbreviation).
'ace' placed around 'r' is 'ACRE'.
(Other definitions for acre that I've seen before include "amount of land" , "0.405 of a hectare" , "Imperial land measurement" , "Muslims here besieged" , "marine location in Israel" .)