Mental health of an utter, all round fool (6)
I believe the answer is:
sanity
'mental health' is the definition.
(I know that sanity is a type of mental health)
'an utter all round fool' is the wordplay.
'an utter' becomes 'say' (to utter something is to speak or say it).
'all round' means one lot of letters goes inside another.
'fool' becomes 'nit' (both can mean a stupid person).
'say' enclosing 'nit' is 'SANITY'.
'of' is the link.
(Other definitions for sanity that I've seen before include "Sound sense" , "Soundness of mind" , "Condition of having a sound mind" , "presence of mind?" , "Balance of mind" .)