Where one might sing anthem's opening line (7)
I believe the answer is:
chancel
'where one might sing anthem's' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'opening line' is the wordplay.
'opening' becomes 'chance' (I've seen this in other clues).
'line' becomes 'l' (used when specifying particular lines from a poem).
'chance'+'l'='CHANCEL'
Can you help me to learn more?
(Other definitions for chancel that I've seen before include "what's behind the screen" , "screened-off area" , "where sanctuary may be found" , "choir and clergy's area" , "space for choir" .)