Annelid worm, used in medicine of old (5)

I believe the answer is:

leech

(Other definitions for leech that I've seen before include "Old physician" , "Blood-sucking worm or parasitical person" , "Archaic word for physician" , "Freeloader" , "Clinging type" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.