Officers mostly keeping horse like a sort of plot? (9)
I believe the answer is:
parabolic
'like a sort of plot?' is the definition.
(like a parabola - type of graph)
'officers mostly keeping horse' is the wordplay.
'officers mostly' becomes 'polic' (I can't explain this - if you can you should believe this answer much more).
'keeping' means one lot of letters goes inside another.
'horse' becomes 'Arab' (Arab is a type of horse).
'polic' enclosing 'arab' is 'PARABOLIC'.
Can you help me to learn more?
(Other definitions for parabolic that I've seen before include "Making a curve" , "Like a type of curve" .)