Superior, negative, but safe advice (6,3)

I believe the answer is:

better not

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'safe advice' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'superior negative' is the wordplay.
'superior' becomes 'better' (better is a kind of superior).
'negative' becomes 'not' (I've seen this in another clue).
'better'+'not'='BETTER NOT'

'but' is the link.

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I've seen this clue in the Evening Standard.

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