Flat, horizontal (5)
I believe the answer is:
level
I believe this clue is a double definition.
'flat' is the first definition.
(I know that flat can be written as level)
'horizontal' is the second definition.
(similar in meaning)
(Other definitions for level that I've seen before include "Floor; horizontal" , "Horizontal plane - a palindrome" , "Neither winning, nor losing" , "Balanced" , "Device ot establish a flat surface" .)