Comedian’s associate got so upset before end of joke (6)
I believe the answer is:
stooge
'comedian's associate' is the definition.
(a stooge is the butt of a comedian's jokes)
'got so upset before end of joke' is the wordplay.
'upset' indicates an anagram.
'before' is a charade indicator (letters next to each other).
'end of' says to take the final letters.
The final letter of 'joke' is 'e'.
'got'+'so'='gotso'
'gotso' with letters rearranged gives 'stoog'.
'stoog'+'e'='STOOGE'
(Other definitions for stooge that I've seen before include "A comedian's stage butt or straight man" , "Yes-man" , "Comedian's foil" , "Comedian's feed" , "Comic's straight man" .)