Unsurpassed rage spoiled assessment (8)
I believe the answer is:
greatest
'unsurpassed' is the definition.
(I've seen this before)
'rage spoiled assessment' is the wordplay.
'spoiled' indicates an anagram.
'assessment' becomes 'test' (test is a kind of assessment).
'rage' is an anagram of 'grea'.
'grea'+'test'='GREATEST'
(Other definitions for greatest that I've seen before include "Best-ever" , "Biggest and best, like Muhammad Ali" , "Get stare from the mightiest" , "Most important!" , "Not to be surpassed" .)