Strength needed when struggling to copy out a number (7)

I believe the answer is:

potency

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'strength' is the definition.
(I know that strength can be written as potency)

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

Can you help me to learn more?

(Other definitions for potency that I've seen before include "Power; pet cony (anag.)" , "Strength - effectiveness" , "Might" , "Great power" , "authority" .)

I've seen this clue in the Evening Standard.

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