Delay one on horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of male horse)
'delay one on' is the wordplay.
'delay' becomes 'stall' (stalling is a kind of delaying).
'one' becomes 'i' (Roman numeral).
'stall'+'i'+'on'='STALLION'
(Other definitions for stallion that I've seen before include "Stud" , "Male steed" , "Male horse lost in atolls" , "Mount" , "Uncastrated adult male horse" .)