Stop before one gets on horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(thesaurus)
'stop before one gets on' is the wordplay.
'stop' becomes 'stall' (stalling is a kind of stopping**).
'before' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'gets' is a charade indicator (letters next to each other).
'stall'+'i'+'on'='STALLION'
(Other definitions for stallion that I've seen before include "Mare's mate" , "Male steed" , "Uncastrated adult male horse" , "Male horse lost in atolls" , "Stud" .)