Relative in Spain agreed on jewellery (7)
I believe the answer is:
sibling
'relative' is the definition.
(sibling is a type of relative)
'in spain agreed on jewellery' is the wordplay.
'in spain agreed' becomes 'si' ('yes' in Spanish).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'jewellery' becomes 'bling' (informal term for jewellery).
'si'+'bling'='SIBLING'
(Other definitions for sibling that I've seen before include "Bro, perhaps" , "Brother or sister by the same parents" , "Maybe brother" , "Bro or sis" , "Close relative" .)