Not one used neon (4)
I believe the answer is:
none
'not one' is the definition.
(I've seen this before)
'used neon' is the wordplay.
'used' indicates anagramming the letters.
'neon' anagrammed gives 'NONE'.
(Other definitions for none that I've seen before include "Not a thing or person" , "hour of prayer" , "Not a single person" , "Not even one" , "0?" .)