Here are the possible solutions for used as a reference clue
Answer
Clue
Relevancy
indexes
Clue
Reference aids in reference books
Definition
INDEX, to provide with an index
tableofcontents
Clue
Reference reference
cited
Clue
Used as a reference
Definition
CITE, to quote as an authority or example
drewon
Clue
Used as a reference
obeli
Clue
Daggers used as reference marks in printing.
Definition
OBELUS, (Latin) a symbol used in ancient manuscripts
oed
Clue
Reference where you might find out that a philobat is used in psychoanalysis for a person who enjoys the challenge of coping alone with dangerous situations