Set off to see river crossed by Pooh's pal (7)
I believe the answer is:
trigger
'set off' is the definition.
(synonyms)
'river crossed by pooh's pal' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'crossed by' is an insertion indicator (inserted letters are crossed or covered by others).
'pooh's pal' becomes 'tigger' (I've seen this in another clue).
'r' inserted inside 'tigger' is 'TRIGGER'.
'to see' is the link.
(Other definitions for trigger that I've seen before include "One pulled to fire" , "Equine film star" , "Gun release mechanism" , "Start" , "Component of gun" .)