What does "clue" mean?

[Rebecca_Me]

English Language Tests, Intermediate level

ESL/EFL Test #273 "When do you say this?", question 4

I say "I haven't a clue" when I ..........

(a) am curious
(b) understand nothing
(c) am apologetic

English Language Tests, Intermediate level

ESL/EFL Test #273 "When do you say this?", answer 4

I say "I haven't a clue" when I understand nothing.

Correct answer: (b) understand nothing
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what´s the word clue?

Rebecca_Me

[Conchita]

A clue is a piece of information that helps you solve a mystery.

The set expression 'I haven't a clue' means 'I don't know at all',