Happens to mention, when rentals decrease (4,4)

I believe the answer is:

lets drop

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'happens to mention' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'rentals decrease' is the wordplay.
'rentals' becomes 'lets' (I can't justify this - if you can you should give a lot more credence to this answer).
'decrease' becomes 'drop' (drop is a kind of decrease).
'lets'+'drop'='LETS DROP'

'when' acts as a link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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