At one time (4)
'at one time' is the definition.
(both can mean 'in the past')
This is the entire clue.
(Other definitions for once that I've seen before include "In the old days" , "Cone (anag.)" , "in days of yore?" , "Formerly; as soon as" , "Formerly - not repeated" .)
'at one time' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for formerly that I've seen before include "In the past" , "At one time" , "Previously, before this time" , "Hitherto" .)