The third sign of the zodiac (6)
I believe the answer is:
gemini
'the third sign of the zodiac' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gemini that I've seen before include "Sign of the Twins ( 6)" , "Third zodiacal sign" , "May person" , "Heavenly sign of the Twins" , "Twins star sign" .)