Utter idiot goes in for mental health (6)
I believe the answer is:
sanity
'for mental health' is the definition.
(sanity is a kind of mental health)
'utter idiot goes in' is the wordplay.
'utter' becomes 'say' (to utter something is to speak or say it).
'idiot' becomes 'nit' (nit can mean a stupid person).
'goes in' means one lot of letters goes inside another.
'say' going around 'nit' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Soundness of mind" , "Sound sense" , "Rationality" , "Rational state" , "Reason, condition of being compos mentis" .)