Relative peace ultimately in Riviera city (5)
I believe the answer is:
niece
'relative' is the definition.
(I know that relative can be written as niece)
'peace ultimately in riviera city' is the wordplay.
'ultimately' indicates one should take the final letters.
'in' means one lot of letters goes inside another.
'riviera city' becomes 'nice' (I've seen this in another clue).
The final letter of 'peace' is 'e'.
'e' going into 'nice' is 'NIECE'.
(Other definitions for niece that I've seen before include "One of the family" , "Kinswoman" , "Female family member" , "Brother's girl" , "Criminal" .)