Fish weight brought back for fair (3,3)

I believe the answer is:

not bad

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'fair' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for not bad that I've seen before include "Negative sort of praise, fairly good" , "Informally good, a bit grudging" , "Quite good" , "Reasonably good in negative way" , "Middling (so good?)" .)

I've seen this clue in The Times and The Sunday Times.

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