Polish front of sideboard with rag? (9)
I believe the answer is:
sandpaper
'polish' is the definition.
(thesaurus)
'front of sideboard with rag?' is the wordplay.
'front of' suggests taking the first letters.
'with' becomes 'and'.
'rag?' becomes 'paper' (rag is a kind of paper).
The initial letter of 'sideboard' is 's'.
's'+'and'+'paper'='SANDPAPER'
(Other definitions for sandpaper that I've seen before include "Woodworker's abrasive" , "Material used for smoothing" , "Abrasive smoother" , "Smooth" , "Make smoother" .)