Muse has to leave for day (3)
I believe the answer is:
era
'day' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for era that I've seen before include "Period of alteration" , "Important historical period" , "Eon" , "Series of years" , "generation, maybe" .)