Eighth sign of the zodiac (7)
I believe the answer is:
scorpio
'eighth sign of the zodiac' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scorpio that I've seen before include "7 [SIGN OF THE ZODIAC]" , "Zodiacal sign" , "A heavenly body" , "Astrological sign" , "star group" .)