Idiot can upset a humorist (6)
I believe the answer is:
nitwit
'idiot' is the definition.
(I've seen this before)
'can upset a humorist' is the wordplay.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'upset' says the letters should be written backwards.
'a humorist' becomes 'wit' (wit is a kind of humorist).
'tin' back-to-front is 'nit'.
'nit'+'wit'='NITWIT'
(Other definitions for nitwit that I've seen before include "Plonker" , "Twin it with a right eejit" , "Silly person (colloq.)" , "Simpleton" , "Informally, a foolish person, featherbrain" .)