Answer: once

AnswerCrossword Clue
ONCE".. bitten..."
ONCEThis is often enough
ONCESingle shot
ONCELouise or Turner
ONCEIt's not enough, per Jackie Susann
ONCELong ago
ONCE"Falling Slowly" musical
ONCE"I'm only gonna say this ... ..."
ONCEAs soon as
ONCEHow often you can lose your virginity
ONCE"... upon a time"
ONCEStory starter
ONCEIn the past
ONCEFirst word of "The Raven"
ONCEA long time ago
ONCEEver
ONCEFairy tale opener
ONCENot a second time
ONCEFairy tale's opener
ONCENever again?
onceat some unspecified past time, esp. a long time ago
ONCEIn the old days
ONCEEnough, often
ONCEFormerly
ONCEYears ago
ONCE..-over (quick look)
ONCE"The Raven" opener
ONCE".. bitten, ..."
ONCE"... bitten, twice shy"
ONCEA single time
ONCEFairy tale start
ONCEChildren's-story starter
ONCEOn a single occasion
ONCETale starter
ONCEWord in many limericks
ONCEFairy tale starter
ONCEFairy-tale start
ONCEFirst word in fairy tales
ONCESecond word of a limerick
ONCEStory opening