Not a horse running in shape (7)
I believe the answer is:
nonagon
'shape' is the definition.
(nine-sided shape)
'not a horse running' is the wordplay.
'not' becomes 'no' (negative).
'a horse' becomes 'nag' (term for an old horse).
'running' becomes 'on' (eg when a machine is running it is on).
'no'+'nag'+'on'='NONAGON'
'in' is the link.
(Other definitions for nonagon that I've seen before include "Nine-sided polygon" , "Figure with nine, twenty-one Dn" , "Figure with nine sides" , "Nine-sided figure" , "regular figure" .)