Shorter son goes missing? Don't mention it! (3,2,3)

I believe the answer is:

not at all

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'don't mention it' is the definition.
(both can mean 'you're welcome')

I don't understand the remainder of the clue.

(Other definitions for not at all that I've seen before include "My pleasure" , "denial" , "You're welcome" , "Anything but!" , "acceptance of apology" .)

I've seen this clue in The Independent.

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