Gaseous element unknown in French — or Norwegian originally (5)
I believe the answer is:
xenon
'gaseous element' is the definition.
(I've seen this in another clue)
'unknown in french or norwegian originally' is the wordplay.
'unknown' becomes 'x' (algebra).
'in french' becomes 'en' ('in' in French).
'originally' says to take the initial letters.
The initial letters of 'or norwegian' is 'on'.
'x'+'en'+'on'='XENON'
(Other definitions for xenon that I've seen before include "Noble gas used in lamps" , "Gaseous element, Xe" , "Gas used in bright lamps" , "member of nobility?" , "One of the noble gasses" .)