Left as upset about headliner beginning to question (5,5)
I believe the answer is:
false start
'beginning to question' is the definition.
(a questionable beginning)
'left as upset about headliner' is the wordplay.
'upset' is an anagram indicator.
'about' means one lot of letters goes inside another.
'headliner' becomes 'star'.
'left'+'as'='leftas'
'leftas' with letters rearranged gives 'falset'.
'falset' enclosing 'star' is 'FALSE START'.
(Other definitions for false start that I've seen before include "In which someone jumps the gun" , "In which runner is seen to have jumped the gun" , "Unsuccessful beginning" , "may lead to total recall" .)