Soundness (6)
'soundness' is the definition.
(I have seen 'Soundness of mind' mean 'sanity' so perhaps 'soundness' could also mean 'sanity')
This is all the clue.
(Other definitions for sanity that I've seen before include "Sound powers of mind" , "Rational state" , "Condition of having a sound mind" , "Rationality" , "Soundness of mind" .)
'soundness' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for health that I've seen before include "Clinician's concern - toasted condition?" , "Freedom from illness" , "Good condition" , "Well being; a toast" , "Soundness; toast" .)
(Other definitions for credibility that I've seen before include "Convincing quality" , "Trustworthiness" , "Talent for being convincing" , "Quality of being believable" .)