The accumulation of excess fat in the body (7)
I believe the answer is:
obesity
'the accumulation of excess fat in the body' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for obesity that I've seen before include "health issue" , "State of unhealthy fatness" , "State of being unhealthily fat" , "Excessive and unhealthy fatness" , "Bulk" .)