Weed killer that lasts for years (3)

I believe the answer is:

hoe

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

'weed killer' is the definition.
(I've seen this in another clue)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for hoe that I've seen before include "Tool; promontory" , "Gardening tool" , "Garden implement" , "Garden tool for scraping up weeds" , "Purple gem" .)

I've seen this clue in the Evening Standard.

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.