sth. that helps s.o. find a way out of a difficult situation or solve a difficult problem: Ariadne' thread.
According to Greek myth, Ariadne, a daughter of Minos of Crete, gave Theseus a ball of thread, by which he traced his way out of the labyrinth.
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.