Brightest star in Scorpio (7)
I believe the answer is:
'brightest star in scorpio' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for antares that I've seen before include "Red supergiant star in Scorpio; nae star (anag.)" , "Star reddened" , "RS [RED STAR]" , "brilliant point" , "Red supergiant star in Scorpius" .)