Possible Clues:

• As against fifty to invoice this, perhaps
• Catamaran initially in the dock, one lost to noted female?
• That's a lowdown woman, by the sound of her
• Singer against changing lot
• Against another lot giving voice
• Continued over Italian bridge, losing one singer
• Voice changing at control
• 'Lowdown lady, by the sound of her (9)'
• Drawl finally nailed by actor, not bad voice
• Party music perhaps raised, left to singer