A form of address to a fellow lacks hint of thoughtfulness — one is offensive (6)
I believe the answer is:
abuser
'one is offensive' is the definition.
(a abusive or offensive person)
I can't explain the rest of the clue.
(Other definitions for abuser that I've seen before include "One who reviles or a misuser of drugs" , "one who uses bad language" , "Bully" , "Person who over-indulges in, say, drugs or drink" , "One mistreating another" .)