Expert holding right for so much land (4)
I believe the answer is:
acre
'so much land' is the definition.
(an acre is a large amount of land)
'expert holding right' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'holding' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'ace' enclosing 'r' is 'ACRE'.
'for' is the link.
(Other definitions for acre that I've seen before include "4840 sq yds" , "Israeli seaport" , "Unit of land measurement" , "Unit of area - Israeli port" , "amount of land" .)