Set off to be an expert ticket seller (5,3)
I believe the answer is:
start out
'set off' is the definition.
('start out' can be a synonym of 'set off')
'expert ticket seller' is the wordplay.
'expert' becomes 'star' (star is a kind of expert).
'ticket seller' becomes 'tout' (I've seen this before).
'star'+'tout'='START OUT'
'to be an' is the link.
(Other definitions for start out that I've seen before include "Go off" , "Set off" , "hit the road" , "Take off" , "launch" .)