Bygone days (4)
I believe the answer is:
past
'bygone days' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for past that I've seen before include "Second visitor in 5 [A CHRISTMAS CAROL]" , "Gone by in time" , "tense" , "The time that has elapsed" , "Bygone" .)