Reason fool blocks authority (6)
I believe the answer is:
sanity
'reason' is the definition.
(I've seen this before)
'fool blocks authority' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'blocks' indicates putting letters inside (the inside letters form a 'blockage').
'authority' becomes 'say' (I've seen this in other clues).
'nit' inserted inside 'say' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Rational behaviour" , "Mental health" , "Sound sense" , "Rational state" , "Mental normality" .)