Diamond edging for rich decoration on vehicle (7)
I believe the answer is:
rhombus
'diamond' is the definition.
(I know that diamond can be written as rhombus)
'edging for rich decoration on vehicle' is the wordplay.
'edging for' says to hollow out the word (remove centre letters) (only the letters at the edges).
'decoration' becomes 'om' (I've seen this in other clues).
'on' means one lot of letters go next to another.
'vehicle' becomes 'bus' (bus is a kind of vehicle).
'rich' with its middle removed is 'rh'.
'rh'+'om'+'bus'='RHOMBUS'
(Other definitions for rhombus that I've seen before include "Four-sided figure" , "Type of parallelogram" , "Diamond shape" , "Regular parallelogram" , "Equilateral 3" .)