Stumble on the journey north? (4,2)
I believe the answer is:
trip up
'stumble' is the definition.
(I know that stumble can be written as trip up)
'on the journey north?' is the wordplay.
'on' means one lot of letters go next to another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'journey north?' becomes 'ripup' (I can't explain this - if you can you should believe this answer much more).
't' put next to 'ripup' is 'TRIP UP'.
Can you help me to learn more?
(Other definitions for trip up that I've seen before include "fluff" , "'Cause to stumble (4,2)'" , "Alpine excursion, perhaps" , "Go wrong" , "Blunder" .)