Crossword Genius

On reaching journey’s end, stumble upon (4,4)


I believe the answer is:

trip over

I'm a little stuck... Click here to teach me more about this clue!

'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?

I've seen this clue in the Evening Standard.
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