Old object is on the way out (4)
I believe the answer is:
exit
'way out' is the definition.
(synonyms)
'old object is on the' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'object' becomes 'i' (I is an example).
'is on' says to put letters next to each other.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'ex'+'i'+'t'='EXIT'
(Other definitions for exit that I've seen before include "Departure of actor" , "Withdrawal" , "walk away" , "Sudden rush of pleasure" , "Relinquish the lead" .)