Possible Clues:

• Bass ___ (four-stringed instrument)
• One of a family of stringed instruments
• Instrument that's a homophone of 69-Down
• Instrument, one featured in short book
• Pre-violin instrument played with a bow
• Limited volume restricting one old instrument
• The double bass is the closest modern relative of this old instrument
• Classical sextet uncovered folk instrument
• Cello's kin
• Bowed baroque instrument