Modifies modern-day apartments (6)
I believe the answer is:
adapts
'modifies' is the definition.
(I've seen this before)
'modern-day apartments' is the wordplay.
'modern day' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'apartments' becomes 'apts' (I can't explain this - if you can you should give a lot more credence to this answer).
'ad'+'apts'='ADAPTS'
Can you help me to learn more?
(Other definitions for adapts that I've seen before include "Makes suitable for a new use" , "Converts to another usage" , "Fits" , "Makes fit or tailors to another purpose" , "Changes to fit new circumstances" .)