Twisting brown river untouched by civilisation (7)
I believe the answer is:
natural
'untouched by civilisation' is the definition.
The definition suggests a singular noun which matches the answer.
'twisting brown river' is the wordplay.
'twisting' shows that the letters should be reversed in order (letters twist back).
'brown' becomes 'tan' (tan is a kind of brown).
'river' becomes 'ural' (Russian river).
'tan' backwards is 'nat'.
'nat'+'ural'='NATURAL'
Can you help me to learn more?
(Other definitions for natural that I've seen before include "Unaffected - musical note" , "Inherent" , "as one might expect" , "Organic" , "Unforced" .)