Discotheque effect (6)
I believe the answer is:
strobe
'discotheque effect' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for strobe that I've seen before include "Flash on and off" , "Flashing-light device" , "Flashing lamp" , "High-intensity flashing beam of light" , "Protestant reformer" .)