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Overthrow argument about Murphy, say (5)

Ross

I believe the answer is:

tuber

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This might be a double definition.

'overthrow argument' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

'murphy say' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'about' is the link.

Can you help me to learn more?

(Other definitions for tuber that I've seen before include "Fleshy underground part of plant" , "Rhizome, as of potato" , "Fleshy underground root" , "Brute (anag)" , "Underground stem, root" .)

I've seen this clue in The Telegraph.
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