Being bland is slightly offensive (9)
I believe the answer is:
tasteless
This clue could be a double definition.
'being bland' is the first definition.
('tasteless' can be similar in meaning to 'bland')
'slightly offensive' is the second definition.
'tasteless' can be an answer for 'offensive' (thesaurus). I am not sure about the 'slightly' bit.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for tasteless that I've seen before include "Lacking flavour or aesthetic appreciation" , "Insipid - vulgar" , "Lacking flavour or ability to appreciate the beautiful" , "Lacking in aesthetic judgement" , "Insipid - inappropriate - dull" .)