Hold up one on a horse (8)
I believe the answer is:
stallion
'a horse' is the definition.
(I know that stallion is a type of male horse)
'hold up one on' is the wordplay.
'hold up' becomes 'stall' (stall can mean to delay or hold up).
'one' becomes 'i' (Roman numeral).
'stall'+'i'+'on'='STALLION'
(Other definitions for stallion that I've seen before include "Mare's mate" , "Mount" , "Male horse kept for breeding" , "Stud" , "Uncastrated adult male horse" .)