Crossword Genius

Turning knob - to disprove ... (5)

Ross

I believe the answer is:

rebut

'to disprove' is the definition.
(I know that rebut is a more specific form of the action confute)

'turning knob' is the wordplay.
'turning' shows that the letters should be reversed in order.
'knob' becomes 'tuber' (swelling plant).
'tuber' back-to-front is 'REBUT'.

(Other definitions for rebut that I've seen before include "Claim or prove to be false" , "Contradict or disprove" , "explode" , "Refute - disprove" , "Tuber (anag.)" .)

I've seen this clue in The Independent.
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