Possible Clues:

• Most unhappy person that may be seen on building site
• Building material down street on edge of site
• Relative of slate
• By the sound of it, see you, Esso, for such as 15 down
• Soundly see you, Esso, for a spray of sapphire
• See you, Esso, for the sapphire, perhaps
• Building material making the person most unhappy
• See you, Esso, for a sapphire
• See you, Esso, for the sound of this
• See you, Esso, for 19 across, perhaps