A short reassurance that one doesn't mind? (3,2,3)

I believe the answer is:

not at all

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

'short reassurance that one doesn't mind?' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for not at all that I've seen before include "Never!" , "denial" , "acceptance of apology" , "In no way" , "Don't mention it!" .)

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

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.