Assistant ref's upset with anecdote (11)
I believe the answer is:
confederate
'assistant' is the definition.
(I know that confederate is a type of assistant)
'ref's upset with anecdote' is the wordplay.
'upset with' is an anagram indicator.
'ref' with its letters anagrammed with 'anecdote' gives 'CONFEDERATE'.
(Other definitions for confederate that I've seen before include "Accomplice, fellow worker" , "US Civil War fighter" , "Ally" , "Of the southern states in the US civil war" , "In alliance" .)