Old solver's checking supplement in? (3,3)

I believe the answer is:

not bad

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'old' is the definition.
(I know that old can be written as not bad)

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

Can you help me to learn more?

(Other definitions for not bad that I've seen before include "Grudging praise" , "Quite good" , "Negative sort of praise, fairly good" , "fair" , "Middling (so good?)" .)

I've seen this clue in The Telegraph.

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