Singer in church alternatively performing rites (9)
I believe the answer is:
chorister
'singer' is the definition.
(singer in a choir)
'church alternatively performing rites' is the wordplay.
'church' becomes 'ch' (abbreviation on maps etc).
'alternatively' becomes 'or'.
'performing' indicates anagramming the letters (the letters move around or 'perform').
'rites' with letters rearranged gives 'ister'.
'ch'+'or'+'ister'='CHORISTER'
'in' acts as a link.
(Other definitions for chorister that I've seen before include "Boy in church" , "Choir member" , "Boy singing" , "Choirboy or girl" , "Church, cathedral singer" .)