How you can make trainees train tenants (7)

I believe the answer is:

lessens

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'tenants' is the definition.
Although both the answer and definition are verbs in their -s form, I can't see how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for lessens that I've seen before include "Belittles" , "Disparages" , "dies down" , "Abates" .)

I've seen this clue in The Times and The Sunday Times.

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