Riviera city accommodating eastern relative (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'riviera city accommodating eastern' is the wordplay.
'riviera city' becomes 'nice' (I've seen this in another clue).
'accommodating' means one lot of letters goes inside another.
'eastern' becomes 'e'.
'nice' enclosing 'e' is 'NIECE'.
(Other definitions for niece that I've seen before include "girl further down the line" , "Brother's girl" , "member of different generation" , "Daughter of a sibling" , "Female relation" .)