Possible Clues:

• String section member
• Fiddled with this to a large extent
• Musical instrument held between the knees
• Musical instrument like large violin held between the knees
• Julian Lloyd Webber's instrument
• With bars on round where there are strings attached (5)
• Orchestra member
• Related to fiddle that has nothing to do with what's in Mountjoy
• Ma holds one
• Instrument like large violin held between the knees