Comet alternative (4)
I believe the answer is:
ajax
'comet alternative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ajax that I've seen before include "Greek hero of the Trojan war - Dutch football team" , "classic war hero" , "Amsterdam football team" , "Greek hero of the Trojan War - Dutch soccer team" , "Hero of Greek mythology" .)