Send forth spell that reverses (4)
I believe the answer is:
emit
'send forth' is the definition.
(I've seen this before)
'spell that reverses' is the wordplay.
'spell' becomes 'time' (spell can mean a period of time).
'that reverses' says the letters should be written in reverse.
'time' back-to-front is 'EMIT'.
(Other definitions for emit that I've seen before include "Give off or discharge" , "Discharge - utter" , "Produce and discharge" , "Issue" , "Exhale" .)