Speak about fool’s ability to think straight (6)
I believe the answer is:
sanity
'ability to think straight' is the definition.
I can't tell whether this definition defines the answer.
'speak about fool's' is the wordplay.
'speak' becomes 'say' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'fool' becomes 'nit' (both can mean a stupid person).
'say' going around 'nit' is 'SANITY'.
Can you help me to learn more?
(Other definitions for sanity that I've seen before include "Reason and coherence" , "Reason, condition of being compos mentis" , "Rationality" , "State of having a healthy mind" , "Sound powers of mind" .)