Smooth singer's first with rubbish Queen (9)
I believe the answer is:
sandpaper
'smooth' is the definition.
(to smooth with sandpaper)
'singer's first with rubbish queen' is the wordplay.
'singer's first' becomes 's' (1st letter of 'singer').
'with' becomes 'and'.
'rubbish' becomes 'pap' (pap can mean rubbish or trivia).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
's'+'and'+'pap'+'er'='SANDPAPER'
(Other definitions for sandpaper that I've seen before include "Make smoother" , "Means of smoothing wood" , "Abrasive smoother" , "Woodworker's abrasive" , "Abrasive material" .)