Transport British relative to get a gem (9)
I believe the answer is:
carbuncle
'a gem' is the definition.
(I know that carbuncle is a type of gem)
'transport british relative' is the wordplay.
'transport' becomes 'car' (car is a method of transport).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'relative' becomes 'uncle' (uncle is a kind of relative).
'car'+'b'+'uncle'='CARBUNCLE'
'to get' acts as a link.
(Other definitions for carbuncle that I've seen before include "eyesore architecturally" , "Severe abscess in the skin" , "Skin eruption - gemstone" , "Large boil-like inflammation - gemstone" , "Architectural eyesore" .)