Go too far past stretch of river (9)
I believe the answer is:
overreach
'go too far' is the definition.
(to overreach is to try to do more than is possible)
'past stretch of river' is the wordplay.
'past' becomes 'over' (I've seen this in another clue).
'stretch of river' becomes 'reach' (reach can mean a stretch of a river).
'over'+'reach'='OVERREACH'
(Other definitions for overreach that I've seen before include "Get the better" , "go too far" , "stretch too far on a ladder?" , "Fail by aiming too high" .)