Article on The Mikado’s opening song (6)
I believe the answer is:
anthem
'song' is the definition.
(anthem is a kind of song)
'article on the mikado's opening' is the wordplay.
'article' becomes 'an' (indefinite article).
'on' says to put letters next to each other.
'opening' says to take the initial letters.
The first letter of 'mikado' is 'm'.
'an'+'the'+'m'='ANTHEM'
(Other definitions for anthem that I've seen before include "Church choral piece" , "Internationale?" , "Uplifting song -- the man (anag)" , "National hymn" , "Song of praise or loyalty" .)