Unwanted discharge upsetting many (4)
I believe the answer is:
snot
'unwanted discharge' is the definition.
(discharge from one's nose)
'upsetting many' is the wordplay.
'upsetting' shows that the letters should be reversed in order.
'many' becomes 'tons' (I've seen this in another clue).
'tons' backwards is 'SNOT'.
(Other definitions for snot that I've seen before include "Mucus" , "Dog" , "Contemptible type" .)