Figure twelve worn by horse (7)
I believe the answer is:
nonagon
'figure' is the definition.
(shape or figure with 9 sides)
'twelve worn by horse' is the wordplay.
'twelve' becomes 'noon' (I've seen this before).
'worn by' is an insertion indicator (I've seen this in other clues).
'horse' becomes 'nag' (term for an old horse).
'noon' placed around 'nag' is 'NONAGON'.
(Other definitions for nonagon that I've seen before include "Nine-sided shape" , "Nine sided figure" , "Nine-sided polygon" , "regular figure" , "Nine-sided figure" .)