clue 1 
(kl)
(kl)n.
Something that serves to guide or direct in the solution of a problem or mystery.
tr.v. clued, clue·ing or clu·ing, clues
To give (someone) guiding information: Clue me in on what's happening around the office.
[Variant of CLEW1 (from Theseus's use of a ball of thread as a guide through the Cretan labyrinth).]