Bill about a land measure (4)
I believe the answer is:
Here is my best explanation:
'a land measure' is the definition.
(I've seen this before)
'bill about' is the wordplay.
'bill' becomes 'ac'.
'about' becomes 're' (regarding).
(Other definitions for acre that I've seen before include "BATTLE", "Mediterranean port", "Race (anag.)", "Care about the piece of land", "Unit of land measurement", "Land unit", "point four nought five of a hectare", "Four thousand, eight hundred and forty square yards", "Imperial land measure" and "in Western Brazil".)