Highly reliable scan (10)

I believe the answer is:

ultrasound

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'scan' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'highly reliable' is the wordplay.
'highly' becomes 'ultra' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'reliable' becomes 'sound' (I've seen this in another clue).
'ultra'+'sound'='ULTRASOUND'

Can you help me to learn more?

(Another definition for ultrasound that I've seen is " diagnostic aid?".)

I've seen this clue in The Independent.

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