You hear this again and again - no pets in apartments (6)
I believe the answer is:
mantra
'you hear this again and again no pets in apartments' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for mantra that I've seen before include "Buddhist aid to concentration" , "Much-repeated phrase" , "Sacred words used as incantation" , "repeating phrase" , "Repeated sacred word or words" .)