Possible Clues:

• Thin girl with stringed instrument
• The time when something ceases to have validity
• Water down cement mix used by inspector
• Thin out, make less strong
• Thin copper instrument
• By the sound of it, die with its music so that it's no longer neat
• Zestful lairds missing the odd turn, having less concentration
• Add water to liquid
• Girl on instrument is not neat
• Thin down by adding water