Musical part that you get with Prokofiev but not with Mendelssohn (7)
I believe the answer is:
secondo
'musical part that you get with prokofiev' is the definition.
'secondo' can be an answer for 'musical part' (the lower part in a duet). I'm not certain of the rest of the definition.
'not with mendelssohn' is the wordplay.
'not' becomes 'o' (I've seen this in other clues).
'with' means one lot of letters go next to another.
'mendelssohn' becomes 'second' (I can't justify this - if you can you should give a lot more credence to this answer).
'o' put after 'second' is 'SECONDO'.
'but' is the link.
(Other definitions for secondo that I've seen before include "lower part" , "Part of duet" , "Part played by duettist" , "Work hands perform" , "Musical" .)