Piece of Bach, perhaps: leave it alone to start with (7)
I believe the answer is:
partita
'piece of bach perhaps' is the definition.
(partita is a type of piece of music)
'leave it alone to start with' is the wordplay.
'leave' becomes 'part' (parting means leaving or separating).
'to start with' indicates taking the first letters.
The initial letter of 'alone' is 'a'.
'part'+'it'+'a'='PARTITA'
(Other definitions for partita that I've seen before include "set of musical compositions" , "noted composition" , "(Musical) suite" , "Instrumental suite" , "musical piece" .)