"Take the First Step" is a best seller (5,3)
I believe the answer is:
start out
'take the first step' is the definition.
(I've seen this in another clue)
'best seller' is the wordplay.
'best' becomes 'star' (star can mean the best or most excellent).
'seller' becomes 'tout' (tout is a kind of seller).
'star'+'tout'='START OUT'
'is a' acts as a link.
(Other definitions for start out that I've seen before include "hit the road" , "Embark (on a venture)" , "Set off" , "Go off" , "launch" .)