Arc lamp gas (5)
I believe the answer is:
xenon
'arc lamp gas' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for xenon that I've seen before include "One of the noble set" , "Inert gas, Xe" , "Noble element" , "Gaseous element, Xe" , "Inert gas used in radio valves" .)