Fatness (7)
I believe the answer is:
obesity
'fatness' is the definition.
(I know that obesity is a type of fatness)
This is all the clue.
(Other definitions for obesity that I've seen before include "Condition of excessive fatness" , "Corpulence" , "State of unhealthy fatness" , "Abnormal fatness" , "State of unhealthy overweight" .)