An insult but not behind one's back! (7)
I believe the answer is:
affront
'an insult' is the definition.
('affront' can be a synonym of 'insult')
'not behind one's back' is the wordplay.
I cannot really understand how this works, but
an anagram of 'not' is 'ont' which is located in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
This explanation may well be incorrect...
'but' acts as a link.
Can you help me to learn more?
(Other definitions for affront that I've seen before include "Rant off over insult" , "Indignity" , "not trivial offence" , "Cause of outrage or offence" , "provocation" .)