Spell "issue” in reverse (4)
I believe the answer is:
time
'spell' is the definition.
(spell can mean a period of time)
'issue in reverse' is the wordplay.
'issue' becomes 'emit' (both can mean to give out or radiate).
'in reverse' says the letters should be written backwards.
'emit' back-to-front is 'TIME'.
(Other definitions for time that I've seen before include "Era" , "> moment" , "It's done in prison" , "weekly magazine" , "The father of Truth, presumably" .)