Get going with best-performing ticket seller (5,3)
I believe the answer is:
start out
'get going' is the definition.
(I've seen this in another clue)
'with best-performing ticket seller' is the wordplay.
'with' says to put letters next to each other.
'best-performing' becomes 'star' (as in a star player).
'ticket seller' becomes 'tout' (as in a ticket tout).
'star'+'tout' is 'START OUT'.
(Other definitions for start out that I've seen before include "hit the road" , "launch" , "Embark (on a venture)" , "Take off" , "Go off" .)