Corrupt, not quite so unsubstantiated (8)

I believe the answer is:

baseless

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'quite so unsubstantiated' is the definition.
'baseless' can be an answer for 'unsubstantiated' (I've seen this in another clue). I am not sure about the 'quite so' bit.

'corrupt not' is the wordplay.
'corrupt' becomes 'base'.
'not' becomes 'less' (I've seen this in other clues).
'base'+'less'='BASELESS'

Can you help me to learn more?

(Other definitions for baseless that I've seen before include "lacking support" , "Without foundation, unsupported" , "Without grounds" , "Without foundation, unjustified" , "Lacking foundation, as rumours may" .)

I've seen this clue in The Daily Express.

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