Article filling an opening for music, church music (6)
I believe the answer is:
anthem
'church music' is the definition.
(anthem is a kind of church music)
'article filling an opening for music' is the wordplay.
'article' becomes 'the' (the definite article).
'filling' means one lot of letters goes inside another.
'opening for' suggests taking the first letters.
The initial letter of 'music' is 'm'.
'an'+'m'='anm'
'the' going within 'anm' is 'ANTHEM'.
(Other definitions for anthem that I've seen before include "National tune" , "Song of praise or loyalty" , "The man for a rousing song" , "Song in praise of a nation" , "Choral work" .)