Crossword Genius

A lightning fast exposure? (5)

Ross

I believe the answer is:

flash

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'a lightning' is the definition.
(thesaurus)

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

Can you help me to learn more?

(Other definitions for flash that I've seen before include "Scintillate" , "Momentary brightness" , "Mo" , "Brief burst of light" , "Conspicuously display" .)

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