Old and regularly unsteady (4)
I believe the answer is:
Here is my best explanation:
'old' is the definition.
('used' can be similar in meaning to 'old')
'regularly unsteady' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'unsteady' are 'USED'.
'and' is the link.