Reassessed his rector as a singer (9)
I believe the answer is:
chorister
'a singer' is the definition.
(singer in a choir)
'reassessed his rector' is the wordplay.
'reassessed' is an anagram indicator (I've seen 'reassessing' mean this).
'his'+'rector'='hisrector'
'hisrector' with letters rearranged gives 'CHORISTER'.
'as' acts as a link.
(Other definitions for chorister that I've seen before include "Boy in church" , "Choirboy or girl" , "Choir member" , "Boy singing" , "Boy singer" .)