Before modern medicine, they bled (7)

I believe the answer is:

leeches

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'before modern medicine they' is the definition.
The definition suggests a plural noun which matches the answer.

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

Can you help me to learn more?

(Other definitions for leeches that I've seen before include "Doctors once used them to bleed patients" , "hard to shake off?" , "drains resources" , "Physicians once" , "Bloodsucking parasites" .)

I've seen this clue in the Evening Standard.

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