Speak about idiot making sense (6)
I believe the answer is:
sanity
'sense' is the definition.
(I have seen 'Good sense ' mean 'sanity' so perhaps 'sense' could also mean 'sanity')
'speak about idiot' is the wordplay.
'speak' becomes 'say' (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'idiot' becomes 'nit' (I've seen this before).
'say' enclosing 'nit' is 'SANITY'.
'making' acts as a link.
(Other definitions for sanity that I've seen before include "Mental normality" , "Mental health" , "Rational condition" , "Sound sense" , "Rational behaviour" .)