Leave old computer technology (4)
I believe the answer is:
exit
'leave' is the definition.
(synonyms)
'old computer technology' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'computer technology' becomes 'it' (abbreviation for information technology).
'ex'+'it'='EXIT'
(Other definitions for exit that I've seen before include "Means of egress" , "Actor's departure" , "Way to go!" , "Depart" , "Outlet" .)