A dash just before take-off (5,4)

I believe the answer is:

minus sign

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'a dash' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I don't see?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for minus sign that I've seen before include "indication of negativity?" , "indicating deduction!" , "Subtraction indicator" , "Symbol indicating subtraction" , "Maths operator" .)

I've seen this clue in the Evening Standard.

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