In the past, at this straightaway (4)
I believe the answer is:
once
I believe this is a double definition.
'in the past' is the first definition.
(once upon a time)
'at this straightaway' is the second definition.
('at once' means straightaway)
(Other definitions for once that I've seen before include "One-time - one time" , "Quick examination" , "In former times" , "... upon a time for the storyteller" , "In the old days" .)