Happens to mention, when rentals decrease (4,4)
I believe the answer is:
Here is my best explanation:
'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).
'when' acts as a link.