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Spell
as
part
of
school
learning
(6)
Spell as part of school learning (6)
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The whole clue is the definition (& lit or double definition).
How the clue works
This is a double definition. A Spell of time is a PERIOD, and a part of school learning — a lesson slot on the timetable — is also a PERIOD. The word "as" simply links the two definitions.