Midday, and horse is in shape (7)
I believe the answer is:
nonagon
'shape' is the definition.
(nine-sided shape)
'midday and horse is in' is the wordplay.
'midday' becomes 'noo' (I can't explain this - if you can you should believe this answer much more).
'and' becomes 'n' (common abbreviation for 'and').
'horse' becomes 'nag' (term for an old horse).
'is in' means one lot of letters goes inside another.
'noo'+'n'='noon'
'noon' placed around 'nag' is 'NONAGON'.
Can you help me to learn more?
(Other definitions for nonagon that I've seen before include "Figure with nine, twenty-one Dn" , "Nine sided figure" , "regular figure" , "Nine-sided shape" , "Nine-sided polygon" .)