Old and regularly unsteady (4)
I believe the answer is:
used
'old' is the definition.
(as in used or second-hand items)
'regularly unsteady' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'unsteady' are 'USED'.
'and' is the link.
(Other definitions for used that I've seen before include "Enjoyed" , "Was treating" , "Worn" , "was milking" , "Exploited group" .)