Clue  —n. 1 fact or idea that serves as a guide, or suggests a line of inquiry, in a problem or investigation. 2 piece of evidence etc. In the detection of a crime. 3 verbal formula as a hint to what is to be inserted in a crossword. —v. (clues, clued, cluing or clueing) provide a clue to.  clue in (or up) slang inform. Not have a clue colloq. Be ignorant or incompetent. [var. Of old english clew]