A Definition Clue occurs when the writer describes the meaning of a word immediately following its use.  Often a brief definition of, a definition clue, or a synonym for a word is included in the sentence that the student is reading.
If the meaning is not formally defined, then the writer will use commas, parentheses, or dashes to indicate that a definition clue is to follow.
 

Examples:

Discrimination is making a difference in treatment on a basis other than individual merit.

Germanium, a grayish white hard chemical element used as a semiconductor, is necessary for our Chemistry experiment.