Gem displayed by English composer in sports car (6)
I believe the answer is:
garnet
'gem' is the definition.
(I know that garnet is a type of transparent gem)
'english composer in sports car' is the wordplay.
'english composer' becomes 'Arne' (Thomas Arne).
'in' means one lot of letters goes inside another.
'sports car' becomes 'GT' (abbreviation for grand tourer).
'arne' inserted inside 'gt' is 'GARNET'.
'displayed by' acts as a link.
(Other definitions for garnet that I've seen before include "Deep red gem stone" , "Precious stone, usually red" , "Deep red gemstone" , "Hard glassy mineral used as a gemstone" , "Semi-precious stone of deep transparent red" .)