Brightness shown by English in old Welsh county briefly (5)

I believe the answer is:

gleam

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'brightness' is the definition.
(I know that gleam is a type of effulgence)

I don't understand the rest of the clue.

(Other definitions for gleam that I've seen before include "Brief bright light" , "Glisten" , "Shine brightly" , "Moderate glow" , "Sparkle, glimmer" .)

I've seen this clue in The Times.

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