Lead fireball (4)
I believe the answer is:
star
This clue is a double definition.
'lead' is the first definition.
('star' can be a synonym of 'lead')
'fireball' is the second definition.
(Other definitions for star that I've seen before include "Cult sci-fi film" , "Possible Christmas tree topper?" , "One at the top of the bill" , "Principal actor in a film" , "Famous person" .)