Accidental (4)
'accidental' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for flat that I've seen before include "Personal residence" , "Pied-a-terre" , "Apartment - on the level" , "Even - somewhere to live" , "without qualification" .)
'accidental' is the definition.
(synonyms)
This is all the clue.
(Other definitions for casual that I've seen before include "Informal or blase" , "displaying lack of commitment" , "Insouciant" , "Informal or not showing commitment" , "Informal or happening by chance" .)