Map detail — it describes three main points (5)
I believe the answer is:
inset
'map detail' is the definition.
(I've seen this in another clue)
'it describes three main points' is the wordplay.
'describes' means one lot of letters goes inside another (describe can mean to form the outline of).
'three main points' becomes 'nse' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' enclosing 'nse' is 'INSET'.
Can you help me to learn more?
(Other definitions for inset that I've seen before include "Placed inside" , "subsidiary diagram" , "Small picture placed within a larger one" , "Page within a page" , "Small map within another" .)