Idle line stops working (9)
I believe the answer is:
pointless
'idle' is the definition.
(thesaurus)
'line stops working' is the wordplay.
'working' is an anagram indicator.
'line'+'stops'='linestops'
'linestops' with letters rearranged gives 'POINTLESS'.
(Other definitions for pointless that I've seen before include "Futile - blunt" , "no avail" , "Serving no purpose" , "Quite vain" , "having scored zero" .)