Turned up with barge: river an easy challenge (8)
I believe the answer is:
pushover
'an easy challenge' is the definition.
(something which is easy to do)
'turned up with barge river' is the wordplay.
'turned' shows that the letters should be reversed in order.
'with' says to put letters next to each other.
'barge' becomes 'shove' (I've seen this before).
'river' becomes 'r'.
'up' in reverse letter order is 'pu'.
'pu'+'shove'+'r'='PUSHOVER'
(Other definitions for pushover that I've seen before include "A thing easily done" , "Something easily done" , "No challenge" , "breeze" , "One is easily defeated or outwitted" .)