Fishy means of measuring? (5)

I believe the answer is:

scale

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'fishy means' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

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

Can you help me to learn more?

(Other definitions for scale that I've seen before include "Tones rising maybe" , "Size ratio; skin flake" , "little plate" , "Relative magnitude" , "plaque" .)

I've seen this clue in the Evening Standard.

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