Pressed into service, is old (4)
I believe the answer is:
used
I believe this clue is a double definition.
'pressed into service' is the first definition.
(I've seen this in another clue)
'old' is the second definition.
(as in used or second-hand items)
'is' is the link.
(Other definitions for used that I've seen before include "Enjoyed" , "with some traffic" , "took a chance on becoming dependent" , "Worn" , "Pre-owned" .)