Break is critical on northward journey (4)
I believe the answer is:
snap
'break' is the definition.
(snapping is a kind of breaking)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for snap that I've seen before include "Break sharply like dry twig" , "Quick photograph - impression" , "game of cards" , "Informal photograph" , "you may take on holiday" .)