Examiner of idiots, upset no end (8)
I believe the answer is:
assessor
'examiner' is the definition.
(I've seen this before)
'idiots upset no end' is the wordplay.
'idiots' becomes 'asses' (I've seen this before).
'upset' becomes 'sore' (I've seen this in other clues).
'no end' means to remove the last letter.
'sore' with its last letter taken off is 'sor'.
'asses'+'sor'='ASSESSOR'
'of' acts as a link.
(Other definitions for assessor that I've seen before include "One who judges" , "Legal adviser" , "Person who estimates values" , "One who estimates or fixes value" , "One making evaluation" .)