Crossword Genius

Balance very hard with old plough (9)

Ross

I believe the answer is:

steelyard

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'balance' is the definition.
(I know that steelyard is a type of balance)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for steelyard that I've seen before include "Weighing machine" , "Portable balance for weighing items" .)

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