Lack of activity unsettling antagonist (10)
I believe the answer is:
stagnation
'lack of activity' is the definition.
(I've seen this before)
'unsettling antagonist' is the wordplay.
'unsettling' is an anagram indicator.
'antagonist' is an anagram of 'STAGNATION'.
(Other definitions for stagnation that I've seen before include "Lack of activity" , "Nothing much happens" , "Inactivity" , "It's still" , "making no progress" .)