Society girl knight rejected? Rubbish! (6)
I believe the answer is:
debris
'rubbish' is the definition.
(debris is a kind of rubbish)
'society girl knight rejected?' is the wordplay.
'society girl' becomes 'deb' (I've seen this before).
'knight' becomes 'sir' (I've seen this before).
'rejected?' says the letters should be written backwards.
'sir' reversed gives 'ris'.
'deb'+'ris'='DEBRIS'
(Other definitions for debris that I've seen before include "aftermath of wild reception?" , "Brides (anag) - bits and pieces" , "Rubble, lots of broken pieces" , "Rocky fragments" , "Broken remains" .)