Article by poet a slow piece (7)
I believe the answer is:
andante
'a slow piece' is the definition.
(slow piece of music)
'article by poet' is the wordplay.
'article' becomes 'an' (English indefinite article).
'by' says to put letters next to each other.
'poet' becomes 'dante' (13th- and 14th-century Italian poet).
'an'+'dante'='ANDANTE'
(Other definitions for andante that I've seen before include "moving unhurriedly" , "(Of tempo) moderately slow" , "moving steadily" , "progressing slowly" , "(Musically) at a moderate pace" .)