Not serious, time or distance (9)

I believe the answer is:

lightyear

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'distance' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'not serious time' is the wordplay.
'not serious' becomes 'light' ('light' is the opposite to 'serious').
'time' becomes 'year' (year is a unit of time).
'light'+'year'='LIGHTYEAR'

'or' is the link.

Can you help me to learn more?

(Another definition for lightyear that I've seen is " a very long way".)

I've seen this clue in The Daily Express.

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