A river curve? (3)
I believe the answer is:
arc
'a river curve?' is the definition.
'arc' can be an answer for 'curve?' (arc is a kind of curve). I'm not sure about the 'a river' bit.
'a river curve?' is the wordplay.
'river curve?' becomes 'rc' (this could be a standard abbreviation which I don't know about).
'a'+'rc'='ARC'
Can you help me to learn more?
(Other definitions for arc that I've seen before include "development of plot" , "A curve; a discharge" , "Semi-circle" , "section of Circle Line" , "bends?" .)