Pieces of land one reserves (4)
I believe the answer is:
ares
'pieces of land' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'one reserves' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'reserves' becomes 'res' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'res'='ARES'
Can you help me to learn more?
(Other definitions for ares that I've seen before include "Land measures" , "Greek god of war" , "War-god" , "figure of worship" , "Belligerent god" .)