Twelve keeping horse in shape (7)
I believe the answer is:
nonagon
'shape' is the definition.
(nine-sided shape)
'twelve keeping horse' is the wordplay.
'twelve' becomes 'noon' (twelve noon).
'keeping' is an insertion indicator (some letters kept inside).
'horse' becomes 'nag' (term for an old horse).
'noon' going around 'nag' is 'NONAGON'.
'in' is the link.
(Other definitions for nonagon that I've seen before include "geometric figure" , "Figure with nine sides" , "Nine-sided polygon" , "Nine-sided shape" , "Nine-sided figure" .)