A fool in country flats (5)
I believe the answer is:
units
'flats' is the definition.
(I know this)
'a fool in country' is the wordplay.
'a fool' becomes 'nit' (both can mean a stupid person).
'in' indicates putting letters inside.
'country' becomes 'us' (U.S. is an example).
'nit' inserted within 'us' is 'UNITS'.
(Other definitions for units that I've seen before include "Single undivided wholes" , "Single parts of a whole" , "Measures of alcohol" , "Individual things, like pieces of furniture" , "military teams" .)