Or richest variation for singer (9)
I believe the answer is:
chorister
'singer' is the definition.
(singer in a choir)
'or richest variation' is the wordplay.
'variation' indicates an anagram.
'or'+'richest'='orrichest'
'orrichest' with letters rearranged gives 'CHORISTER'.
'for' acts as a link.
(Other definitions for chorister that I've seen before include "Boy in church" , "Choir member" , "Boy singing" , "Boy singer" , "Choirboy or girl" .)