Mental health of utter fool inside (6)
I believe the answer is:
sanity
'mental health' is the definition.
(sanity is a kind of mental health)
'utter fool inside' is the wordplay.
'utter' becomes 'say' (saying is a kind of uttering**).
'fool' becomes 'nit' (I've seen this before).
'inside' indicates putting letters inside.
'say' enclosing 'nit' is 'SANITY'.
'of' acts as a link.
(Other definitions for sanity that I've seen before include "Reason and coherence" , "Soundness of mind" , "Healthy mental state" , "Good sense" , "Mental wellness" .)