Best or otherwise, pulsating light (6)
I believe the answer is:
strobe
'pulsating light' is the definition.
(I've seen this in another clue)
'best or otherwise' is the wordplay.
'otherwise' is an anagram indicator.
'best'+'or'='bestor'
'bestor' with letters rearranged gives 'STROBE'.
(Other definitions for strobe that I've seen before include "Flashing disco light" , "Sister to Hansel" , "Flashing lamp" , "Sort of lighting" , "Flash on and off" .)