Singer, or first to interrupt singer (9)
I believe the answer is:
chorister
'singer' is the definition.
(chorister is a kind of singer)
'or first to interrupt singer' is the wordplay.
'first to' becomes 'ist' (I am not sure about this - if you are sure you should believe this answer much more).
'interrupt' indicates putting letters inside.
'singer' becomes 'cher' (I've seen this in other clues).
'or'+'ist'='orist'
'orist' inserted within 'cher' is 'CHORISTER'.
Can you help me to learn more?
(Other definitions for chorister that I've seen before include "Church, cathedral singer" , "Boy singing" , "Choirboy or girl" , "Boy singer" , "Boy in church" .)