Formerly, once (4)
I believe the answer is:
I believe this is a double definition.
'formerly' is the first definition.
(archaic term for formerly or long ago)
'once' is the second definition.
(archaic term for formerly or long ago)
(Other definitions for erst that I've seen before include "At first; formerly" , "in the past" , "in former times" , "Rest turned formerly" .)