Answer:

was

AnswerCrossword Clue
WASFunctioned as
WASThird word in many limericks
WASWord in obituaries
WASSee 24-Across
WASUsed to be
WASThird word of many a limerick
WASActed as
WASThird word of many limericks
WASLived
WASNever-... (unsuccessful sort)
WASExisted
WASHas been
WASLived and breathed
WASPerformed the role of
WASIs past?
WASIs in the past
WASHad a life
WASIsn't anymore
WASTook place
WASServed as
WASVerb for a historian
WASOccurred
WASIs in the past?
WASIs no longer
WASIs in another form?
WASBreathed
WASWalked the earth
WASThird word in a limerick
WASHad been
WASIsn't now
WASOnce existed