Answer: invisibleink

AnswerCrossword Clue
INVISIBLEINKSecret-message stuff
INVISIBLEINKSecret message medium
INVISIBLEINKIt's used by many a junior spy
INVISIBLEINK-
INVISIBLEINKEnd of the quip
INVISIBLEINKPing shears
INVISIBLEINKMedium for secret messages
INVISIBLEINKSecretive contradiction