No-one doubts what this signal means (3,5)

I believe the answer is:

all clear

I'm a little stuck... Click here to teach me more about this clue!

'signal means' is the definition.
'all clear' can be an answer for 'signal' (I know that all clear is a type of signal). I am not certain of the 'means' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for all clear that I've seen before include "unlike 21 [GREY AREAS] ?" , "End-of-danger signal" , "I understand everything" , "no further questions needed" , "cue for renewed action" .)

I've seen this clue in The Globe and Mail.

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