Crossword Genius

To the reader, means “indeterminate periods of time” (6)

Ross

I believe the answer is:

spells

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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