Reportedly weaken instruction (6)

I believe the answer is:

lessen

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'reportedly weaken' is the definition.
'lessen' can be an answer for 'weaken' (lessening is a kind of weakening). I'm not certain of the 'reportedly' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for lessen that I've seen before include "decline" , "Abate" , "Cut back or diminish" , "Make fewer in number" , "Make smaller for lenses" .)

I've seen this clue in The Mirror.

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