Rivals set a mule free (8)
I believe the answer is:
emulates
'rivals' is the definition.
(emulate can mean to equal or rival someone)
'set a mule free' is the wordplay.
'free' indicates anagramming the letters.
'set'+'a'+'mule'='setamule'
'setamule' is an anagram of 'EMULATES'.
(Other definitions for emulates that I've seen before include "Tries hard to be equal to" , "Copies" , "Imitates" , "tries to follow" .)