Gas in some lasers (5)
I believe the answer is:
argon
'gas in some lasers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for argon that I've seen before include "rare gas" , "Gas, inert constituent of air" , "Inert gas used in light bulbs" , "Inert gas, present in small amounts in the air" , "elementary" .)