Dye plant marks snake (6)

I believe the answer is:

madder

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'dye' is the definition.
(thesaurus)

'marks snake' is the wordplay.
'marks' becomes 'M' (former German currency).
'snake' becomes 'adder' (type of snake).
'm'+'adder'='MADDER'

'plant' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for madder that I've seen before include "Angrier at the red colour" , "Red dye; more barmy" , "More disturbed - plant" , "Plant; more angry" , "Plant; more insane" .)

I've seen this clue in The Guardian.

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