London borough initially ignored rubbish outside old plant (9)
I believe the answer is:
arrowroot
'plant' is the definition.
(arrowroot is a kind of plant)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for arrowroot that I've seen before include "Starchy plant" , "Medicinal plant" , "culinary powder" , "Carib foodstuff" , "Starch used for thickening sauces etc" .)