From IFWiki

A piece of information within the natural prose of the game intended to stop a player from using knowledge that the player character does not have, and to verbally nudge the player in the direction of the solution to the puzzle. Clues tend to be strategically placed within a game so that the player finds them when they are needed most to advance the story.

An example of a clue is to find a password or to notice that a particular object, place, or person is mentioned, therefore insinuating a connection between character and clue.

See also hint.