Group in open red omnibus, backward-looking in fashion (6)
I believe the answer is:
modern
'in fashion' is the definition.
(I've seen this before)
'group in open red omnibus backward-looking' is the wordplay.
'group in' says the answer is hidden in the clue (a group of letters hidden in the clue).
'backward-looking' shows that the letters should be reversed in order.
'nredom' is hidden within 'open red omnibus'.
'nredom' written backwards gives 'MODERN'.
(Other definitions for modern that I've seen before include "See 11" , "Novel" , "Present-day" , "Belonging to the present or recent times" , "'Contemporary, up-to-date (6)'" .)