Well, why are they complaining, perhaps you can provide more details? This would depend on how you might want to address it i.e., -
1) Not cleaned prior to the arrival. Why haven’t they said anything before now?
2) They expect it to be cleaned during their stay?
3) What exactly is not clean or led to this complaint?
Not cleaned prior to check-in - why have they waited/ lived in apartment for 6 days before saying anything; due to the time lapse, this scenario sounds like an attempt to knock off their bill, I would dispute this.
Cleaning during their stay - did you tell them it would be cleaned whilst they are in occupancy? Briefly looking at your listing, there is no mention of this although there is a cleaning fee. The cleaning fee is normally towards cost of cleaning up after they check out i.e., I have a cleaning fee which contributes to the cleaner who attends after each guest checks out to clean all areas they used, wash the bedding and towels, steam the pillows and mattress etc.; the guest are expected to clean up after themselves during their stay.
Finally - have have they actually said is unclean about the apartment? Is it likely or reasonable?
I think your course of action depends on what exactly they said, the above are just examples. Whatever you do, ensure you are communication on the Airbnb platform regarding this matter. If Airbnb need to get involved down the line, they will not accept text messages, WhatsApp, personal e-mail etc. as acceptable evidence of communion.
BW,
Ama