Possessing reason for one to accommodate fool (6)
I believe the answer is:
sanity
'possessing reason' is the definition.
(being sane is having reason)
'for one to accommodate fool' is the wordplay.
'for one' becomes 'say' (both can mean 'for example').
'to accommodate' is an insertion indicator.
'fool' becomes 'nit' (I've seen this before).
'say' placed around 'nit' is 'SANITY'.
(Other definitions for sanity that I've seen before include "Balance of mind" , "Mental normality" , "State of having a healthy mind" , "Good sense" , "Condition of having a sound mind" .)