Do send back credit notes - they're offensive (6)
I believe the answer is:
odious
'offensive' is the definition.
(I've seen this in another clue)
'do send back credit notes' is the wordplay.
'send back' says the letters should be written in reverse (I've seen 'sends back' mean this).
'credit notes' becomes 'ious' (I am not sure about this - if you are sure you should believe this answer much more).
'do' written backwards gives 'od'.
'od'+'ious'='ODIOUS'
'they're' acts as a link.
Can you help me to learn more?
(Other definitions for odious that I've seen before include "Really hateful, abhorrent" , "Replusive, abhorrent (6)" , "Unequivocally detestable" , "Loathsome and repulsive" , "Vile" .)