Chart mapping out twelve houses (6)
I believe the answer is:
zodiac
'chart' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for zodiac that I've seen before include "Heavenly belt with 12 signs" , "astrological study" , "Houses seen on a cycle" , "Imaginary belt of heavenly signs" , "Heavenly area" .)