Go too far across a stretch of water (9)
I believe the answer is:
overreach
'go too far' is the definition.
(to overreach is to try to do more than is possible)
'across a stretch of water' is the wordplay.
'across' becomes 'over' (synonyms).
'a stretch of water' becomes 'reach' (I can't explain this - if you can you should believe this answer much more).
'over'+'reach'='OVERREACH'
Can you help me to learn more?
(Other definitions for overreach that I've seen before include "go too far" , "stretch too far on a ladder?" , "Fail by aiming too high" , "Get the better" .)