Mean rider upset the rest (9)
I believe the answer is:
remainder
'the rest' is the definition.
('remainder' can be a synonym of 'rest')
'mean rider upset' is the wordplay.
'upset' indicates anagramming the letters.
'mean'+'rider'='meanrider'
'meanrider' is an anagram of 'REMAINDER'.
(Other definitions for remainder that I've seen before include "that's left" , "Balance" , "Rest - sell off (books) cheap" , "Unsold copy of book" , "It's left over after division" .)