Subject about right for a mundane circle (6)
I believe the answer is:
tropic
'mundane circle' is the definition.
'tropic' can be an answer for 'circle' (I've seen this before). I'm not certain of the 'mundane' bit.
'subject about right' is the wordplay.
'subject' becomes 'topic' (as in a topic for discussion).
'about' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'topic' placed around 'r' is 'TROPIC'.
'for a' is the link.
Can you help me to learn more?
(Other definitions for tropic that I've seen before include "Parallel of latitude about 25o N or S" , "Particular latitude" , "Line of latitude determined by the solstice" , "Imaginary circle on globe" , "One of two parallels north and south of the equator" .)