Some backwards diagonal line, that's plain (5)
I believe the answer is:
llano
'that's plain' is the definition.
(a grassy plain)
'some backwards diagonal line' is the wordplay.
'backwards' shows that the letters should be reversed in order.
'some' says the answer is hidden in the clue (some parts of the word are used).
'onall' can be found hidden inside 'diagonal line'.
'onall' written backwards gives 'LLANO'.
(Other definitions for llano that I've seen before include "Flat area" , "(South American) grassy plain" , "Grassy plain in South?America" , "Treeless grassy plain in South America" , "Treeless area" .)