Materials processed in plant (5,4)

I believe the answer is:

mares tail

I'm a little stuck... Click here to teach me more about this clue!

'in plant' is the definition.
'mares tail' can be an answer for 'plant' (I've seen this before). I'm not sure about the 'in' bit.

'materials processed' is the wordplay.
'processed' indicates an anagram.
'materials' anagrammed gives 'MARES-TAIL'.

Can you help me to learn more?

(Other definitions for mares tail that I've seen before include "marsh plant" , "Long streak of cirrus cloud" , "water plant" .)

I've seen this clue in The Guardian.

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