Obstruct one riding horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of male horse)
'obstruct one riding' is the wordplay.
'obstruct' becomes 'stall' (I can't explain this - if you can you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'riding' becomes 'on' (I've seen this in other clues).
'stall'+'i'+'on'='STALLION'
Can you help me to learn more?
(Other definitions for stallion that I've seen before include "Male horse lost in atolls" , "Male horse, often used for breeding" , "Male steed" , "Uncastrated adult male horse" , "Mount" .)