Reveal sordid business was not quite truthful — unhygienic bun riddled with penicillin (4,4,5,5,2,6)
I believe the answer is:
wash ones dirty linen in public
'quite truthful unhygienic bun riddled with penicillin' is the definition.
I can't judge whether this defines the answer.
'reveal sordid business was not' is the wordplay.
I cannot really see how this works, but
'sordid' could be 'dirty' (synonyms) and 'dirty' is located in the answer.
'business' could be 'line' (as in someone's line of work) and 'line' is found within the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for wash ones dirty linen in public that I've seen before include "Expose matters discreditable to one" , "expose family skeleton" , "Use launderette" .)