One sure to succeed Brown going up river (7)
I believe the answer is:
natural
'one sure to succeed' is the definition.
I can't judge whether this defines the answer.
'brown going up river' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'going up' is a reversal indicator (in a down clue, the letters are written upwards).
'river' becomes 'ural' (Russian river).
'tan' in reverse letter order is 'nat'.
'nat'+'ural'='NATURAL'
Can you help me to learn more?
(Other definitions for natural that I've seen before include "An ideal person" , "Unforced" , "Inborn, normal ( 7)" , "character on the staff" , "person regarded as certain to qualify" .)