Flashing light set or not, beep occurring intermittently (6)
I believe the answer is:
strobe
'flashing light' is the definition.
(I've seen this before)
'set or not beep occurring intermittently' is the wordplay.
'occurring intermittently' indicates alternate letters.
'set'+'or'+'not'+'beep'='setornotbeep'
The alternating letters of 'setornotbeep' are 'STROBE'.
(Other definitions for strobe that I've seen before include "Flashing disco light" , "High-intensity flashing beam of light" , "Flash on and off" , "Sister to Hansel" , "Flashing lamp" .)