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 really see how this works, but
'up' could be 'each' (I've seen this in another clue) and 'each' is located in the answer.
'river?' could be 'r' (abbreviation for river) and 'r' is found within the answer.
This accounts for all the letters.
This may be the basis of the 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" .)