Carrying Roo initially, Winnie the Pooh's friend set off (7)
I believe the answer is:
'set off' is the definition.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for trigger that I've seen before include "Lever on gun" , "Roy Rogers' horse" , "Gun release mechanism" , "Activating device" , "Lever to activate firing mechanism" .)