Arranged systematically - and that's flat! (7)
I believe the answer is:
tabular
'that's flat' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for tabular that I've seen before include "in an array" , "Flat" , "(Of data) presented in rows and columns" , "Arranged in rows and columns" .)