On reaching journey’s end, stumble upon (4,4)
I believe the answer is:
'end stumble upon' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'on reaching journey's' is the wordplay.
I cannot really see how this works, but
'on' could be 'over' (I've seen this in other clues) and 'over' is located in the answer.
'journey' could be 'trip' (trip is a kind of journey) and 'trip' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?