It has a supporting role in work by Bach (7)
I believe the answer is:
partita
'work by bach' is the definition.
The definition suggests a singular noun which matches the answer.
'it has a supporting role' is the wordplay.
'has' says to put letters next to each other.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'role' becomes 'part' (eg a part or role in a film).
'it' next to 'a' is 'ita'.
'ita' put after 'part' is 'PARTITA'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for partita that I've seen before include "Instrumental suite" , "musical piece" , "set of musical compositions" , "(Musical) suite" , "Piece of music" .)