Diagnostic aid that'll help you see today's theme? (3,7)

I believe the answer is:

cat scanner

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This might be a double definition.

'diagnostic aid' is the first definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'that'll help you see today's theme?' is the second definition.
The definition suggests a singular noun which matches the answer.

Can you help me to learn more?

I've seen this clue in The Guardian.

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