Old part of clock (6-4)
I believe this clue is a double definition.
'old' is the first definition.
(as in previously belonging to someone else)
'part of clock' is the second definition.
(as in the second and minute hands on a clock)
(Other definitions for second-hand that I've seen before include "Not original", "Not new - part of watch!", "Previously owned - part of watch", "Used".)