Flat — level (5)
I believe the answer is:
plane
I believe this is a double definition.
'flat' is the first definition.
(I know that flat can be written as plane)
'level' is the second definition.
(I know that plane is a type of level)
(Other definitions for plane that I've seen before include "Smoother" , "Tree - figure - aircraft" , "Smooth" , "Form of transport" , "Nepal (anag.)" .)