Answer: pre

AnswerCrossword Clue
PRECautionary beginning?
PREBefore, when before
PREAnte- kin
PRESelf-descriptive sort of fix?
PREEqual of ante-
PREPost- opposite
PREPrefix's prefix
PREAnte- alternative
PRE...-K
PRECautionary opening?
PRESeason opener?
PREOpposite of post-
PREDisposition opener
PREExisting intro
PRELead-in for law or med
PREBefore, at the fore
PRECambrian opening
PREExisting start
PREHistoric leader?
PREMed lead-in
prePetroleum Refining Engineer
PREFab start?
PREJudicial start?
PREHistoric introduction?
PREHistoric starter
PRELaw or school lead-in
PREPost opposite
PREPost's opposite
PRE..-op
PREPrefix for vent or sent
PRESeason lead-in
PREAmble start
PREPost-'s opposite
PREStart to dominate?
PRE..-Columbian era
PREHistoric beginning?
PREBefore
PREBeginning for sent or vent
PRE..-med
PRE"Before," when before