Presumably a short deprecatory remark (3,2,3)

I believe the answer is:

not at all

'deprecatory remark' is the definition.
(deprecatory or modest phrase)

I can't explain the rest of the clue.

(Other definitions for not at all that I've seen before include "You're welcome" , "My pleasure" , "denial" , "By no means - polite disclaimer" , "Never!" .)

I've seen this clue in The Guardian.

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