A stretch up the river? (5)
I believe the answer is:
reach
'a stretch' is the definition.
('reach' can be a synonym of 'stretch')
'up the river?' is the wordplay.
I cannot quite understand how this works, but
'river?' could be 'ea' (I've seen this in other clues) and 'ea' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for reach that I've seen before include "Attain - continuous extent of water" , "Open stretch of water" , "Stretch of a river between two bends" , "It's an advantage for a boxer" , "Grasp" .)