Horse's breaking down, lame at best (10)
I believe the answer is:
stablemate
'horse's' is the definition.
(stablemate is a kind of horse)
'breaking down lame at best' is the wordplay.
'breaking down' indicates an anagram.
'lame'+'at'+'best'='lameatbest'
'lameatbest' with letters rearranged gives 'STABLEMATE'.
(Other definitions for stablemate that I've seen before include "Blames Tate (anag) - person from the same club" , "someone from same place" , "One from the same organisation as another" , "colleague in same organization" , "person in same outfit" .)