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 the rest of the clue.

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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