To retire is no longer the thing (4)
I believe the answer is:
exit
'to retire' is the definition.
(thesaurus)
'no longer the thing' is the wordplay.
'no longer' becomes 'ex'.
'the thing' becomes 'it'.
'ex'+'it'='EXIT'
'is' acts as a link.
(Other definitions for exit that I've seen before include "Way out sign" , "Withdrawal" , "Way to leave" , "Outlet" , "Sudden rush of pleasure" .)