100ml drained from head swelling (4)

I believe the answer is:

node

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'swelling' is the definition.
(swelling on the body)

I can't explain the rest of the clue.

(Other definitions for node that I've seen before include "Lump at junction" , "Network junction" , "Natural swelling of part of a plant or animal" , "Point of intersection" , "Done to a small lump" .)

I've seen this clue in The Times.

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