Backside mostly elegant and shocking (11)
I believe the answer is:
disgraceful
'shocking' is the definition.
(synonyms)
'backside mostly elegant' is the wordplay.
'backside mostly' becomes 'dis' (I can't justify this - if you can you should believe this answer much more).
'elegant' becomes 'graceful' (associated in meaning).
'dis'+'graceful'='DISGRACEFUL'
'and' acts as a link.
(Other definitions for disgraceful that I've seen before include "Abominable - scandalous" , "far from honourable" , "Shockingly unacceptable" , "Dishonourable" .)