Horse -- stop one then proceed! (8)
I believe the answer is:
stallion
'horse' is the definition.
(thesaurus)
'stop one then proceed' is the wordplay.
'stop' becomes 'stall' (stalling is a kind of stopping**).
'one' becomes 'i' (Roman numeral).
'then' says to put letters next to each other.
'proceed' becomes 'on' (as imperatives both can mean 'go forward').
'stall'+'i'+'on'='STALLION'
(Other definitions for stallion that I've seen before include "Mount" , "Mare's mate" , "Male horse kept for breeding" , "Stud" , "Uncastrated adult male horse" .)