Insubstantial impurity that keeps the stars hidden (5,9)

I believe the answer is:

light pollution

I'm a little stuck... Click here to teach me more about this clue!

'keeps the stars hidden' is the definition.
I can't judge whether this defines the answer.

'insubstantial impurity' is the wordplay.
'insubstantial' becomes 'light' (I've seen this before).
'impurity' becomes 'pollution' (pollution is a kind of impurity).
'light'+'pollution'='LIGHT POLLUTION'

'that' is the link.

Can you help me to learn more?

(Another definition for light pollution that I've seen is " Beaming out of place".)

I've seen this clue in The Daily Express.

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