Formerly, archaically (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
erst
'formerly archaically' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for erst that I've seen before include "Rest turned formerly" , "in former times" , "At first; formerly" , "in the past" .)