Upsetting us, sender's bad manners (8)
I believe the answer is:
rudeness
'bad manners' is the definition.
(I've seen this before)
'upsetting us sender's' is the wordplay.
'upsetting' indicates anagramming the letters.
'us'+'sender'='ussender'
'ussender' with letters rearranged gives 'RUDENESS'.
(Other definitions for rudeness that I've seen before include "Discourtesy" , "loutish behaviour" , "Bad manners, lack of politeness" , "Vulgarity" , "Lip" .)