Top guns circle restricted land units (5)
I believe the answer is:
acres
'land units' is the definition.
(I've seen this before)
'top guns circle restricted' is the wordplay.
'top guns' becomes 'aces' (I've seen this in another clue).
'circle' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'restricted' is an insertion indicator.
'aces' placed around 'r' is 'ACRES'.
Can you help me to learn more?
(Other definitions for acres that I've seen before include "Rival played" , "Cares about the land measures" , "Units of land measurement" , "Imperial units or measures of land" , "farmland maybe?" .)