Similar beasts of burden initially in race? That’s killing! (13)
I believe the answer is:
assassination
'killing' is the definition.
(assassination is a kind of killing)
'similar beasts of burden initially in race?' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is located in the answer.
'in' is within the answer.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for assassination that I've seen before include "(Eg, political) murder" , "Merciful, lenient" , "Murder of a prominent person" , "execution" , "Ordered killing" .)