Outrageous, senior relative in apartment (8)
I believe the answer is:
flagrant
'outrageous' is the definition.
(flagrant can mean outrageous or blatant)
'senior relative in apartment' is the wordplay.
'senior relative' becomes 'gran' (short for grandmother).
'in' is an insertion indicator.
'apartment' becomes 'flat' (synonyms).
'gran' put within 'flat' is 'FLAGRANT'.
(Other definitions for flagrant that I've seen before include "Very bad and obvious, say offence" , "Shocking" , "Unashamed" , "Openly outrageous" , "notorious" .)