Bygone era (4)
I believe the answer is:
yore
'bygone era' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yore that I've seen before include "Old usage for formerly, as in days of. . ." , "Long ago (literary)" , "Time long ago" , "Bygone time" , "Time long past" .)