Excursion set in motion (4)
I believe the answer is:
trip
I believe this is a double definition.
'excursion' is the first definition.
(a trip is an excursion or outing)
'motion' is the second definition.
(trip is a kind of motion)
'set in' acts as a link.
(Other definitions for trip that I've seen before include "day out?" , "Excursion (maybe hallucinatory!)" , "A short journey - stumble on it" , "Journey - cause to stumble" , "Journey, excursion" .)