A lightning fast exposure? (5)
I believe the answer is:
flash
'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" .)