To the reader, means “indeterminate periods of time” (6)
I believe the answer is:
spells
'periods of time' is the definition.
'spells' can be an answer for 'periods' (spell is a kind of period). I am not certain of the 'of time' bit.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for spells that I've seen before include "Signals" , "Periods of times or witche's curses" , "Periods of times or witch's curses" , "Witches cast them" , "Short periods of time" .)