Community (and all associated with it) is comfortable with arrest after case's conclusion (9)
I believe the answer is:
ecosystem
'community and all associated with it' is the definition.
(an ecological community)
'comfortable with arrest after case's conclusion' is the wordplay.
'comfortable' becomes 'cosy' (similar in meaning).
'with' means one lot of letters go next to another.
'arrest' becomes 'stem' (both can mean to stop).
'after' is a charade indicator (letters next to each other).
'conclusion' says to take the final letters.
The last letter of 'case' is 'e'.
'cosy' next to 'stem' is 'cosystem'.
'cosystem' after 'e' is 'ECOSYSTEM'.
'is' is the link.
(Other definitions for ecosystem that I've seen before include "The interaction of a group of organisms with their physical environment" , "integrated community" , "orderly collection of organisms" , "Community of organisms and their environment" , "Biological community with its environment" .)