Days of __; long ago
I believe the answer is:
yore
'days of long ago' 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 "Long ago (poetic)" , "Bygone time" , "Yesteryear" , "Formerly, of long ago" , "Old usage for formerly, as in days of. . ." .)