Fool can upset with humour (6)
I believe the answer is:
nitwit
'fool' is the definition.
(thesaurus)
'can upset with humour' is the wordplay.
'can' becomes 'tin' ('tin' can be a synonym of 'can').
'upset' shows that the letters should be reversed in order.
'with' means one lot of letters go next to another.
'humour' becomes 'wit' ('wit' can be a synonym of 'humour').
'tin' in reverse letter order is 'nit'.
'nit'+'wit'='NITWIT'
(Other definitions for nitwit that I've seen before include "Foolish person like humorous louse" , "Turkey" , "Plonker" , "Blockhead" , "Informally a stupid person, like funny flea?" .)