Offensive letter you read, polite last of all (4)
I believe the answer is:
rude
'offensive' is the definition.
(thesaurus)
'letter you read polite last of all' is the wordplay.
'letter' becomes 'r' ().
'last of all' indicates one should take the final letters.
The final letters of 'you read polite' are 'ude'.
'r'+'ude'='RUDE'
(Other definitions for rude that I've seen before include "'Bad-mannered, discourteous (4)'" , "Offensive; unpolished" , "Blue" , "Uncivilised" , "Ill-mannered; hearty" .)