Brilliant colour displayed by vehicle left in group (7)
I believe the answer is:
scarlet
'brilliant colour' is the definition.
(scarlet is a bright colour)
'vehicle left in group' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'left' becomes 'L' (common abbreviation).
'in' is an insertion indicator.
'group' becomes 'set' (both can mean a collection).
'car'+'l'='carl'
'carl' going into 'set' is 'SCARLET'.
'displayed by' is the link.
(Other definitions for scarlet that I've seen before include "Cherry-red" , "Shade of red" , "Colour of embarrassment?" , "Fallen" , "bloody?" .)