Extremely safe medical scan (10)

I believe the answer is:

ultrasound

'medical scan' is the definition.

'extremely safe' is the wordplay.
'extremely' becomes 'ultra' (prefix meaning extremely).
'safe' becomes 'sound' (similar in meaning).
'ultra'+'sound'='ULTRASOUND'

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

I've seen this clue in The Times.

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