What we may have in summer for a period of instruction (6)

I believe the answer is:

lesson

'period of instruction' is the definition.
(a lesson is instructional)

I cannot understand how the remainder of the clue works.

(Other definitions for lesson that I've seen before include "Spell of instruction" , "Moral" , "warning message" , "What teacher provides" , "Sermon or class session" .)

I've seen this clue in The Times.

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