Answer:

UPON

Answer	Crossword Clue
UPON	After "once"
UPON	At the high point of
UPON	Atop

UPON	Covering
UPON	Fairy tale's second word
UPON	Second word in a fairy tale
UPON	Straddling
UPON	"It came ... a midnight.."
UPON	Once ... a time
UPON	Over
UPON	Second word in many fairy tales

UPON	Riding, say
UPON	"Once ... a midnight dreary"
UPON	Preposition in British place names
UPON	"Once .. a time..."
UPON	".. my word!"
UPON	Savvy about
UPON	At the top of
UPON	Topping

UPON	Fairy-tale preposition
UPON	Shortly after
UPON	At the moment of
UPON	Having studied
UPON	Apprised of
UPON	Well versed in
UPON	Abreast of
UPON	Resting atop

UPON	"Once .. a midnight dreary..."
UPON	"... my word!"
UPON	Berwick-...-Tweed, England
UPON	Conversant about
UPON	Immediately after
UPON	Very soon after
UPON	"Once ....."
UPON	"When You Wish .. a Star"

UPON	Informed about
UPON	It often comes after once
UPON	On top of
UPON	"Once .. a time ..."
UPON	Knowledgeable about