Crass, somewhat bacchanalian affair (4)
I believe the answer is:
naff
'crass' is the definition.
(I've seen this before)
'somewhat bacchanalian affair' is the wordplay.
'somewhat' indicates a hidden word.
'NAFF' is hidden within 'bacchanalian affair'.
(Other definitions for naff that I've seen before include "Vulgar, out of fashion or of poor quality" , "Lacking taste or quality" , "Of dubious taste" , "Vulgarly tasteless" , "Lacking taste or style" .)