Act before the headliner (6)
I believe the answer is:
opener
'act before the headliner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opener that I've seen before include "First part" , "player, first to bat" , "Tins need one" , "Device for accessing contents of tins or bottles" , "(Team's) first batsman" .)