Upset hanger-on by mistake? Nonsense (7)
I believe the answer is:
rubbish
'nonsense' is the definition.
(I've seen this in another clue)
'upset hanger-on by mistake?' is the wordplay.
'upset' is a reversal indicator.
'hanger-on' becomes 'bur' (type of seed which hangs onto clothing).
'by' means one lot of letters go next to another.
'mistake?' becomes 'bish' (I've seen this before).
'bur' back-to-front is 'rub'.
'rub'+'bish'='RUBBISH'
(Other definitions for rubbish that I've seen before include "Stuff and nonsense" , "Awful" , "Nonsense; very bad" , "Worthless material to discard" , "Refuse, junk" .)