Gem of a motor bought initially by relative (9)
I believe the answer is:
Here is my best explanation:
'gem' is the definition.
(carbuncle is a kind of gem)
'a motor bought initially by relative' is the wordplay.
'a motor' becomes 'car' (I've seen this before).
'initially' says to take the initial letters.
'by' is a charade indicator (letters next to each other).
'relative' becomes 'uncle' (uncle is a kind of relative**).
The initial letter of 'bought' is 'b'.
'of' is the link.
(Other definitions for carbuncle that I've seen before include "Gemstone", "cheaper jewel?", "Skin eruption - gemstone", "stone", "Red gem", "Sort of boil; a gem", "Architectural eyesore", "Gem", "Eruption" and "Red gemstone; sort of boil".)