Soundness of mind (6)
I believe the answer is:
sanity
'soundness of mind' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sanity that I've seen before include "Sound sense" , "Rationality" , "Mental wellness" , "Reason, condition of being compos mentis" , "Soundness of mind" .)