To start things rolling, Pooh's friend wheeled around Roo's head (7)
I believe the answer is:
trigger
'to start' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for trigger that I've seen before include "One pulled to fire" , "Gun firing device" , "Initiate" , "Releasing device" , "Pull it to discharge gun" .)