Answer: xis

AnswerCrossword Clue
XIS

Greek letters

XIS

They follow the nus

XIS

Greek consonants

XIS
Omicrons' preceders
XIS
Followers of nus
XISLetters before omicrons
XISThey precede omicrons
XISNus' successors
XISFirst words of the title of Sue Grafton's as-yet-unwritten 24th Kinsey Millhone mystery
XISFraternity letters
XISOmicron preceders
XISFrat letters
XISExpected 2015 Sue Grafton title starter
XISSorority letters
XISGreek letters.
XISBefore omicrons
XISNumbers or letters
XISLetters from Greece
XISAttic letters
XISOmicrons' predecessors
XISLetters from Xanthippe
XISGreek characters
XISCharacters from Sophocles
XISGreek letters after nus
XISLetters after nus
XISNu followers
XISThey follow nus
XISLetters between nus and omicrons
XISSome fraternity row letters