Muse having to leave for day (3)
I believe the answer is:
'for day' is the definition.
(day can mean an era of time)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for era that I've seen before include "Specific time" , "Important date" , "Important historical period" , "Eon" , "Are involved in time span" .)