Sign of illness? (6)
This clue is a double definition.
'sign' is the first definition.
(sign of the zodiac)
'illness?' is the second definition.
(cancer is a kind of health problem)
'of' acts as a link.
(Other definitions for cancer that I've seen before include "Crab star sign" , "Malignant growth and sign of the zodiac" , "Sign of the Crab, or a Tropic" , "Fourth sign of the zodiac" , "A Tropic or a malignant disease" .)
'sign of illness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for symptom that I've seen before include "Sign of illness" , "Indicator of disease" , "It alerts one to illness" , "Evidence for disease" , "Indication (of disease, problem)" .)