Old Norse poems from some unremembered days (4)
I believe the answer is:
edda
'old norse poems' is the definition.
(collection of Norse poems)
'from some unremembered days' is the wordplay.
'from' indicates the answer is hidden within the clue.
'EDDA' can be found hidden inside 'some unremembered days'.
(Other definitions for edda that I've seen before include "Old Icelandic book" , "Ancient Icelandic writings" , "Collection of old songs" , "Norse saga" , "Old tales" .)