Brag about Mercedes-Benz? Pretentious talk! (7)
I believe the answer is:
bombast
'pretentious talk' is the definition.
(I've seen this in another clue)
'brag about mercedes-benz?' is the wordplay.
'brag' becomes 'boast' (to boast is to brag or show off).
'about' indicates putting letters inside.
'mercedes-benz?' becomes 'mb' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'boast' enclosing 'mb' is 'BOMBAST'.
Can you help me to learn more?
(Other definitions for bombast that I've seen before include "Pomposity" , "High-sounding words with little meaning" , "Pompous language" , "verbosity" , "Empty inflated language" .)