Spell "occasion" (4)
I believe the answer is:
time
This clue is a double definition.
'spell' is the first definition.
(spell can mean a period of time)
'occasion' is the second definition.
(I've seen this in another clue)
(Other definitions for time that I've seen before include "It's done in prison" , "Speech to close pub" , "Finite duration" , "Continuum of past, present and future" , "Duration; tempo" .)