Sensationally, if clearly, making speed of light run! (7)

I believe the answer is:

luridly

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'light run' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I can't see?

I can't explain the remainder of the clue.

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(Other definitions for luridly that I've seen before include "In macabre fashion" , "In melodramatic fashion" , "Shockingly" , "In a very sensational manner" .)

I've seen this clue in The Times.

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