To quit is no longer the thing (4)
I believe the answer is:
exit
'to quit' is the definition.
(thesaurus)
'no longer the thing' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'the thing' becomes 'it'.
'ex'+'it'='EXIT'
'is' is the link.
(Other definitions for exit that I've seen before include "[DOOR] ?" , "Escape route?" , "Leave the stage" , "Way out sign" , "Withdrawal" .)