Reason fool imprisoned by state (6)
I believe the answer is:
sanity
'reason' is the definition.
(thesaurus)
'fool imprisoned by state' is the wordplay.
'fool' becomes 'nit' (I've seen this before).
'imprisoned by' indicates putting letters inside.
'state' becomes 'say' ('say' can be a synonym of 'state').
'nit' going within 'say' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Rational condition" , "Good sense" , "Mental health" , "Mental wellness" , "Soundness of mind" .)