Delay one working with a horse (8)
I believe the answer is:
stallion
'horse' is the definition.
(I know that stallion is a type of horse)
'delay one working' is the wordplay.
'delay' becomes 'stall' (stalling is a kind of delaying).
'one' becomes 'i' (Roman numeral).
'working' becomes 'on' (on can mean working or activated).
'stall'+'i'+'on'='STALLION'
'with a' acts as a link.
(Other definitions for stallion that I've seen before include "Uncastrated adult male horse" , "Mare's mate" , "Male horse, often used for breeding" , "Mount" , "Stud" .)