Set off with Pooh's friend round river (7)
I believe the answer is:
trigger
'set off' is the definition.
('trigger' can be a synonym of 'set off')
'pooh's friend round river' is the wordplay.
'pooh's friend' becomes 'tigger' (Winnie the Pooh reference).
'round' means one lot of letters goes inside another.
'river' becomes 'r'.
'tigger' placed around 'r' is 'TRIGGER'.
'with' acts as a link.
(Other definitions for trigger that I've seen before include "Start" , "Releasing device" , "trip" , "Gun release mechanism" , "Equine film star" .)