Shielding with no answer must be offensive (9)
I believe the answer is:
insulting
'offensive' is the definition.
('insulting' can be similar in meaning to 'offensive')
'shielding with no answer' is the wordplay.
'shielding' becomes 'insulating'.
'with no' indicates named letters should be taken away.
'answer' becomes 'a' (abbreviation eg in Q and A).
'insulating' with 'a' taken out is 'INSULTING'.
'must be' acts as a link.
(Other definitions for insulting that I've seen before include "Offensive" , "Treating with disrespect" , "What offender's doing" .)