At one concerning some land (3)
I believe the answer is:
are
'some land' is the definition.
(historical unit of measurement - 100 square metres)
'at one concerning' is the wordplay.
'at' means one lot of letters go next to another.
'one' becomes 'a' (a thing is one thing).
'concerning' becomes 're' (both can mean on a particular topic).
'a' next to 're' is 'ARE'.
(Other definitions for are that I've seen before include "(They/you/we) exist" , "Era (anag.)" , "Is, in the plural" , "live" , "Unit of land measure equal to 100 square metres" .)