Simpleton handed rude letters (10)
I believe the answer is:
dunderhead
'simpleton' is the definition.
(I know that dunderhead is a type of dipstick)
'handed rude letters' is the wordplay.
'letters' indicates an anagram.
'handed'+'rude'='handedrude'
'handedrude' is an anagram of 'DUNDERHEAD'.
(Other definitions for dunderhead that I've seen before include "fool" , "Dunce" , "Ignoramus" , "Stupid person" , "Idiot" .)