Backward about everything, it’s plain (5)
I believe the answer is:
llano
'plain' is the definition.
(thesaurus)
'backward about everything' is the wordplay.
'backward' shows that the letters should be reversed in order.
'about' becomes 'on' (**).
'everything' becomes 'all' (I've seen this before).
'on'+'all'='onall'
'onall' reversed gives 'LLANO'.
'it's' acts as a link.
(Other definitions for llano that I've seen before include "Treeless area" , "Grassy plain in South?America" , "Treeless grassy plain in South America" , "(South American) grassy plain" , "Flat area" .)