Turning knob - to disprove ... (5)
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.)" .)