Brother or sister returned one's flashy jewellery (7)
I believe the answer is:
sibling
'brother or sister' is the definition.
(I've seen this before)
'returned one's flashy jewellery' is the wordplay.
'returned' shows that the letters should be reversed in order.
'one's' becomes 'is' (Roman numeral**).
'flashy jewellery' becomes 'bling' (I've seen this before).
'is' back-to-front is 'si'.
'si'+'bling'='SIBLING'
(Other definitions for sibling that I've seen before include "Maybe brother" , "Bro or sis" , "Bro, perhaps" , "One's brother or sister" , "Close relative" .)