Disco flasher (6)
I believe the answer is:
strobe
'disco flasher' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for strobe that I've seen before include "something flashy" , "Flashing, very bright beam" , "Sister to Hansel" , "Protestant reformer" , "High-intensity flashing beam of light" .)