Instruction not to wear so much (6)

I believe the answer is:

lesson

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

'not to wear so much' is the wordplay.
'not' becomes 'less' (I've seen this in other clues).
'to wear so much' becomes 'on' (I can't explain this - if you can you should believe this answer much more).
'less'+'on'='LESSON'

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(Other definitions for lesson that I've seen before include "Instructive experience" , "Seminar" , "Severe reproof" , "What teacher provides" , "Teaching session or Bible passage" .)

I've seen this clue in The Mirror.

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