It's flat and square (4)
I believe the answer is:
even
I believe this clue is a double definition.
'it's flat' is the first definition.
(similar in meaning)
'square' is the second definition.
(both mean 'owing nothing' as in 'we're all square')
'and' acts as a link.
(Other definitions for even that I've seen before include "like the numbers on two sides here" , "That's not odd" , "Constant" , "Like half the numbers" , "Placid - not easily angered" .)