Extremely cruel guy in control I see (8)

I believe the answer is:

diabolic

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'extremely cruel' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for diabolic that I've seen before include "Extremely cruel" , "wicked" , "Devilish" , "Satanic" , "Fiendish and cruel, like Old Nick" .)

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

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