Take away some land on reserve (8)
I believe the answer is:
subtract
'take away' is the definition.
(thesaurus)
'some land on reserve' is the wordplay.
'some land' becomes 'tract' (I've seen this in other clues**).
'on' is a charade indicator (letters next to each other).
'reserve' becomes 'sub' (I've seen this in other clues).
'tract' put after 'sub' is 'SUBTRACT'.
(Other definitions for subtract that I've seen before include "Take away (a number)" , "dock" , "Remove" , "Knock off" , "Take an amount away" .)