A new article on Mendelssohn’s first choral work (6)
I believe the answer is:
anthem
'choral work' is the definition.
(an anthem might be sung by a choir)
'a new article on mendelssohn's first' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'article' becomes 'the' (the definite article in English).
'on' means one lot of letters go next to another.
'mendelssohn's first' becomes 'm' (1st letter of 'mendelssohn').
'a'+'n'+'the'+'m'='ANTHEM'
(Other definitions for anthem that I've seen before include "Religious chant" , "Hymn of loyalty" , "The man for a rousing song" , "The man for a patriotic song" , "Song in praise of a nation" .)