Smash hits rock endlessly with Queen singer (9)
I believe the answer is:
chorister
'singer' is the definition.
(chorister is a kind of singer)
'smash hits rock endlessly with queen' is the wordplay.
'smash' indicates anagramming the letters.
'endlessly' means to remove the last letter.
'with' means one lot of letters go next to another.
'queen' becomes 'er' (abbreviation for Elizabeth Regina**).
'hits'+'rock'='hitsrock'
'hitsrock' with its last letter taken off is 'hitsroc'.
'hitsroc' with letters rearranged gives 'chorist'.
'chorist'+'er'='CHORISTER'
(Other definitions for chorister that I've seen before include "Choir member" , "Church, cathedral singer" , "Boy singing" , "Choirboy or girl" , "Boy in church" .)