Malignant growth (6)
I believe the answer is:
cancer
'malignant growth' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cancer that I've seen before include "Crab star sign" , "Constellation between Gemini and Leo" , "Malignant growth and sign of the zodiac" , "Fourth sign of the zodiac" , "Zodiac sign of the Crab" .)