Possible Clues:

• At the drop of ___
• At the drop of __
• At the drop of --
• At the drop of —
• At the drop of __ (quickly)
• At the drop of ___ (instantly)
• Answer to the old riddle 'What lies flat when empty, sits up when full?'
• 'I Had ___ When I Came In' (old Irish tune)
• '___ Full of Sky' (2004 Terry Pratchett novel)
• ...drop of ___