An emptier diary for instruction (6)

I believe the answer is:

lesson

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'instruction' is the definition.
(lesson is a kind of instruction)

I can't explain the rest of the clue.

(Other definitions for lesson that I've seen before include "See 1" , "Bible passage read as part of a church service" , "Class session or reading from Bible" , "Spell of instruction" , "Teaching session or Bible passage" .)

I've seen this clue in The Telegraph.

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