Rear end supported by knob (6)
I believe the answer is:
button
'knob' is the definition.
(thesaurus)
'rear end supported by' is the wordplay.
'rear end' becomes 'butt' ('butt' can be a synonym of 'rear end').
'supported by' becomes 'on'.
'butt'+'on'='BUTTON'
(Other definitions for button that I've seen before include "Worthless, they say" , "Round fastener" , "Knob or disc as clothes fastener" , "fast driver?" , "One may invite the press to function" .)