Possible Clues:

• Lttrs tkn frm ths cl
• Quintet seen in late-night hours?
• Series of watering troughs?
• Alphabetical 119-Across
• Run out of rhythm?
• Facetious string?
• Letters found in a so-called 'supervocalic' word
• Group that ends '... and sometimes Y'
• Noteworthy features of rows 1-5, 6-10 and 11-15, in that order
• "___ and sometimes Y"