Some land already claimed, Russians excited, first of all (4)
'some land' is the definition.
(I've seen this before)
'already claimed russians excited first of all' is the wordplay.
'first of all' says to take the initial letters.
The first letters of 'already claimed russians excited' is 'ACRE'.
(Other definitions for acre that I've seen before include "Site of battle", "Old unit of area - old port", "0.405 of a hectare", "Unit of area - Israeli port", "Imperial land measure".)