Judge speed behind a river section (9)
I believe the answer is:
arbitrate
'judge speed' is the definition.
'arbitrate' can be an answer for 'judge' (to arbitrate is to judge or decide). I am not certain of the 'speed' bit.
'behind a river section' is the wordplay.
'behind' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'section' becomes 'bitrate' (I can't explain this - if you can you should give a lot more credence to this answer).
'a' put next to 'r' is 'ar'.
'ar'+'bitrate'='ARBITRATE'
Can you help me to learn more?
(Other definitions for arbitrate that I've seen before include "Decide" , "Act between disputing parties" , "Mediate" , "Adjudge" , "Help settle a dispute" .)