Reason a fool's wearing schoolboy's coat (6)
I believe the answer is:
sanity
'reason' is the definition.
(I've seen this in another clue)
'a fool's wearing schoolboy's coat' is the wordplay.
'fool' becomes 'nit' (both can mean a stupid person).
'wearing' is an insertion indicator.
'coat' suggests removing the centre (only the letters which coat the word are used).
'schoolboy' with its middle taken out is 'sy'.
'a'+'nit'='anit'
'anit' put within 'sy' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Mental health" , "Mental wellness" , "Rationality" , "Balance of mind" , "Good sense" .)