Gardening accessory that's old hat (6)

I believe the answer is:

cloche

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'old hat' is the definition.
(old-fashioned type of hat)

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

(Other definitions for cloche that I've seen before include "Woman's helmet-like hat" , "Cover for garden plants" , "seedlings may need it" , "protection for young stock?" , "useful in the garden" .)

I've seen this clue in The Telegraph, The Guardian and The Globe and Mail.

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