Messiah takes over Queen singer (9)
I believe the answer is:
chorister
'singer' is the definition.
(singer in a choir)
'messiah takes over queen' is the wordplay.
'messiah' becomes 'christ' (Jesus Christ).
'takes' means one lot of letters goes inside another.
'over' becomes 'o' (cricket abbreviation).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'christ' placed around 'o' is 'chorist'.
'chorist'+'er'='CHORISTER'
(Other definitions for chorister that I've seen before include "Church, cathedral singer" , "Choir member" , "Boy singing" , "Boy singer" , "Boy in church" .)