Fourth sign of the zodiac (6)
I believe the answer is:
cancer
'fourth sign of the zodiac' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cancer that I've seen before include "Crab star sign" , "Fourth sign of the zodiac" , "Sign of the Crab, or a Tropic" , "A Tropic or a malignant disease" , "Zodiac sign of the Crab" .)