Tropical tuber (4)

I think it could be either:

Which length description are you looking for?:

yam

'tropical tuber' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for yam that I've seen before include "A starchy tuber" , "May be a sweet spud" , "US sweet potato" , "Potato-like vegetable" , "Edible root" .)

I've seen this clue in The Times and The Telegraph.

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