Works from Sturluson in numbered days (4)
I believe the answer is:
edda
'works from sturluson' is the definition.
(13th century Icelandic book by Snorri Sturluson)
'in numbered days' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EDDA' can be found hidden inside 'numbered days'.
(Other definitions for edda that I've seen before include "book previously" , "One of the two old Icelandic poetic books" , "anthology" , "Book of Icelandic poetry" , "Lots of old lines" .)