Begin as the leading ticket seller? (5,3)
I believe the answer is:
start out
'begin' is the definition.
(synonyms)
'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' is the link.
(Other definitions for start out that I've seen before include "Begin a journey" , "Take off" , "Go off" , "launch" , "hit the road" .)