Disprove brute, running riot (5)

I believe the answer is:

rebut

I'm a little stuck... Click here to teach me more about this clue!

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

'brute running riot' is the wordplay.
'brute' anagrams to 'REBUT'.
Though, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for rebut that I've seen before include "Give the lie to" , "Scotch" , "Prove the claim wrong" , "Brute can refute" , "Refure or disprove" .)

I've seen this clue in The Daily Express.

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