Horse to detest running, losing time (5)
I believe the answer is:
steed
'horse' is the definition.
(steed is a kind of horse)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for steed that I've seen before include "Literary term for horse" , "Horse, mount" , "Powerful horse" , "Trusty horse" , "(Poetic) horse" .)