It involves taking too much interest (5)

I believe the answer is:

usury

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'too much interest' is the definition.
(I've seen this before)

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

(Other definitions for usury that I've seen before include "Loaning money at high interest" , "Exorbitant or unlawful rate of interest" , "Money-lending at very high rates" , "Extortionate lending" , "Shylock's calling" .)

I've seen this clue in the Evening Standard.

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