Element that none used (4)
I believe the answer is:
neon
'element' is the definition.
(I know that neon is a type of element)
'none used' is the wordplay.
'used' indicates anagramming the letters.
'none' anagrammed gives 'NEON'.
'that' acts as a link.
(Other definitions for neon that I've seen before include "Form of lighting" , "Brightly coloured advertising sign" , "Type of lamp" , "Element that gives a red glow in a vacuum tube" , "Gas used in fluorescent lamps" .)