Small prison reported vulnerable spot (4,4)

I believe the answer is:

weak link

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'reported vulnerable spot' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'small prison' is the wordplay.
'small' becomes 'weak' (similar in meaning).
'prison' becomes 'link' (I can't explain this - if you can you should give a lot more credence to this answer).
'weak'+'link'='WEAK LINK'

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I've seen this clue in the Sydney Morning Herald.

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