Powerful heartless tyrant besieging resistance (5)

I believe the answer is:

burly

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'powerful' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for burly that I've seen before include "Muscular and heavily built" , "Stocky" , "Beefy" , "Big and thickset" , "Big and sturdy" .)

I've seen this clue in The Telegraph.

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