Begin as the leading ticket seller? (5,3)
I believe the answer is:
start out
'begin' is the definition.
(I know that begin can be written as start out)
'the leading ticket seller?' is the wordplay.
'the leading' becomes 'star' (I've seen this in another clue).
'ticket seller?' becomes 'tout' (as in a ticket tout).
'star'+'tout'='START OUT'
'as' acts as a link.
(Other definitions for start out that I've seen before include "Begin a journey" , "Take off" , "Go off" , "launch" , "hit the road" .)