Stop one on a horse (8)
I believe the answer is:
stallion
'a horse' is the definition.
(thesaurus)
'stop one on' is the wordplay.
'stop' becomes 'stall' (stalling is a kind of stopping**).
'one' becomes 'i' (Roman numeral).
'stall'+'i'+'on'='STALLION'
(Other definitions for stallion that I've seen before include "Mount" , "Uncastrated adult male horse" , "Male steed" , "Male horse kept for breeding" , "Male horse, often used for breeding" .)