Motor originally bought by relative is gem (9)
I believe the answer is:
carbuncle
'is gem' is the definition.
(I've seen this before)
'motor originally bought by relative' is the wordplay.
'motor' becomes 'car' (informal term for car).
'originally' indicates taking the first letters.
'by' says to put letters next to each other.
'relative' becomes 'uncle' (uncle is a kind of relative).
The first letter of 'bought' is 'b'.
'car'+'b'+'uncle'='CARBUNCLE'
(Other definitions for carbuncle that I've seen before include "Cluster of boils - jewel" , "cheaper jewel?" , "eyesore architecturally" , "National Gallery's Sainsbury Wing?" , "Sort of boil; a gem" .)