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Fish out of river from which scales hang (4)

Ross

I believe the answer is:

beam

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

This clue could be a double definition.

'fish out of river' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'from which scales hang' is the second definition.
(part of a set of scales)

Can you help me to learn more?

(Other definitions for beam that I've seen before include "Ray - large piece of wood" , "Smile, transmit (radio signal)" , "Horizontal structural member" , "A broad smile like lunar ray" , "Smile radiantly" .)

I've seen this clue in The Times.
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