Appalled — but helping to make Branagh a star (6)
I believe the answer is:
aghast
'appalled' is the definition.
(I know that appalled can be written as aghast)
'helping to make branagh a star' is the wordplay.
'helping to make' says the answer is hidden in the clue.
'AGHAST' is hidden in the letters of 'branagh a star'.
'but' is the link.
(Other definitions for aghast that I've seen before include "'Appalled, dismayed (6)'" , "Horror-stricken, filled with consternation" , "Appalled, shocked" , "Horrified, filled iwth consternation" , "struck with fear" .)