I am upset, your setter? I am speechless! (4)
I believe the answer is:
mime
'i am speechless' is the definition.
'i am upset your setter?' is the wordplay.
'i am' becomes 'I'm' (contraction).
'upset' says the letters should be written backwards.
'your setter?' becomes 'me' (the crossword setter of this puzzle).
'im' back-to-front is 'mi'.
'mi'+'me'='MIME'
(Other definitions for mime that I've seen before include "Acting dumb?" , "Job progress" , "Silent performer" , "Imitate as Marcel Marceau does" , "Art of story-telling by body movements" .)