Lack of activity (7)
I believe the answer is:
inertia
'lack of activity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inertia that I've seen before include "Tendency to do nothing or to remain unchanged" , "Disinclination to act" , "Reluctance to change" , "Listlessness" , "Tendency to inaction" .)