Set off brilliant attempt to sell (5,3)
I believe the answer is:
start out
'set off' is the definition.
(synonyms)
'brilliant attempt to sell' is the wordplay.
'brilliant' becomes 'star' (star can mean brilliant eg star performer).
'attempt to sell' becomes 'tout' (I've seen this before).
'star'+'tout'='START OUT'
(Other definitions for start out that I've seen before include "Begin a journey" , "Go off" , "Take off" , "Set off" , "launch" .)