Sign none altered without seriousness (4,5)
I believe the answer is:
neon light
'altered without seriousness' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'sign none' is the wordplay.
I cannot really understand how this works, but
'sign' could be 'light' (light is a kind of sign) and 'light' is present in the answer.
an anagram of 'none' is 'neon' which is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for neon light that I've seen before include "What may shine" , "Illumination used for advertising" , "Advertising sign, perhaps" , "Type of illumination" , "Source of illumination" .)