Sordid, not quite so unfounded (8)
I believe the answer is:
baseless
'quite so unfounded' is the definition.
'baseless' can be an answer for 'unfounded' (synonyms). I am not certain of the 'quite so' bit.
'sordid not' is the wordplay.
'sordid' becomes 'base' (I've seen this in another clue).
'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 foundation, unjustified" , "Lacking foundation, as rumours may" , "With no foundation in fact" .)