Possible Clues:

• When one 2 down, perhaps
• It goeth before the fall
• Sounds like more of 15 across for an adder
• 'Shall I compare thee to a ...'s day?' (Shakespeare)
• Precedes a fall, one might add
• It precedes 66-Across
• Warm season, allegedly
• Mum's with 'er when it's 7 down
• Beam when it's warm?
• One can count up millions in ancient civilisation