Go gaily on a journey (4)
I believe the answer is:
trip
'a journey' is the definition.
(trip is a kind of journey)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for trip that I've seen before include "A short journey - stumble on it" , "Ride" , "Excursion (while on drugs?)" , "Hallucinogenic drug experience - lose one's balance" , "Stumble - excursion" .)