Answer: certain

AnswerCrossword Clue
CERTAINIndubitable
certainagreed upon
certaindefinite but unspecified
certainabsolutely confident
certainwithout a doubt; surely
certainfree from doubt; sure
certaininevitable
CERTAINDefinite
CERTAINQuite sure
CERTAINConfirmed
CERTAINSome nectar I spilled (7)
CERTAINUnquestionable
CERTAINIncontrovertible
CERTAINFoolproof
CERTAINDetermined
CERTAINLike death and taxes
CERTAINPositive.
CERTAINUnfailing.
CERTAINSagan's "A ... smile."
CERTAINReliable.
CERTAINCocksure.
CERTAINIn the bag, so to speak
CERTAINPositive
CERTAINSure
CERTAINSpecified
CERTAINParticular