Relative opulence at last in a French city (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'opulence at last in a french city' is the wordplay.
'at last' says to take the final letters.
'in' is an insertion indicator.
'a french city' becomes 'Nice' (Nice is a French city).
The last letter of 'opulence' is 'e'.
'e' going into 'nice' is 'NIECE'.
(Other definitions for niece that I've seen before include "Young female relative" , "Brother's girl" , "Son's cousin" , "member of different generation" , "Kinswoman" .)