Boxer Liston said to be bright (5)

I believe the answer is:

sunny

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'bright' is the definition.
(similar in meaning)

I cannot understand how the rest of the clue works.

(Other definitions for sunny that I've seen before include "Cheerful; fine (weather)" , "(Of a person's disposition) always positive" , "Bright and cheerful" , "Bright and warm" , "Cheerful - cloudless" .)

I've seen this clue in The Sun.

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