Bottom touching knob (6)
I believe the answer is:
button
'knob' is the definition.
(thesaurus)
'bottom touching' is the wordplay.
'bottom' becomes 'butt' (synonyms).
'touching' becomes 'on'.
'butt'+'on'='BUTTON'
(Other definitions for button that I've seen before include "Jenson --, Formula One driver" , "Electric switch operated by pressing" , "Garment fastener" , "One may invite the press to function" , "On-screen control" .)