UN report not reliable (7)

I believe the answer is:

unsound

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'un report not reliable' is the definition.
I can't tell whether this definition defines the answer.

'not reliable' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'reliable' becomes 'sound' (I've seen this before).
'un'+'sound'='UNSOUND'

Can you help me to learn more?

(Other definitions for unsound that I've seen before include "Unreliable, not well" , "silence?" , "Unfit or unreliable" , "Defective or flawed" , "Imperfect or untenable" .)

I've seen this clue in The Sun.

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