Last pair of gardeners in gardens attack the weeds? That’s a lucky thing (9)

I believe the answer is:

horseshoe

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'lucky thing' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for horseshoe that I've seen before include "One could be lucky" , "Lucky charm" , "Plate nailed to equine hoof" , "Lucky thing" , "Hoof protector" .)

I've seen this clue in The Times.

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