The most common and most likely answer to this crossword clue is the 8 letter word TREKKING
T
R
E
K
K
I
N
G
Buddy explains!

The straight part
We know that "on a great journey" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on a great journey = trekking.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
Crossword Answer definitions
TREKKING verb- To make a slow or arduous journey.
- To journey on foot, especially to hike through mountainous areas.
- To travel by ox wagon.
- Walking in the countryside for pleasure or sport, usually for a longer period of time than for hiking.