After treatment, stay in normal frame of mind (6)
I believe the answer is:
sanity
'normal frame of mind' is the definition.
'after treatment stay in' is the wordplay.
'after treatment' indicates anagramming the letters.
'stay'+'in'='stayin'
'stayin' with letters rearranged gives 'SANITY'.
(Other definitions for sanity that I've seen before include "Reason and coherence" , "Soundness of mind" , "Stay in (anag.)" , "Rational condition" , "presence of mind?" .)