Answer:

disappearingink

Answer	Crossword Clue
DISAPPEARINGINK	?
DISAPPEARINGINK	Secret-message liquid
DISAPPEARINGINK	First trick

DISAPPEARINGINK	-
DISAPPEARINGINK	Code-writer's medium
DISAPPEARINGINK	Gag for a typist

Crossword

a puzzle consisting of a grid of squares and blanks into which words crossing vertically and horizontally are written according to clues.