Points describing radius for circles (7)
I believe the answer is:
tropics
'circles' is the definition.
(I know this)
'points describing radius for' is the wordplay.
'points' becomes 'topics'.
'describing' indicates putting letters inside (describe can mean to form the outline of).
'radius for' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'topics' enclosing 'r' is 'TROPICS'.
Can you help me to learn more?
(Other definitions for tropics that I've seen before include "Cancer, Capricorn" , "torrid zones" , "heated situations" , "Region either side of equator" , "hot regions" .)