Dark vehicle borne by horse (7)
I believe the answer is:
macabre
'dark' is the definition.
(macabre means dark or mysterious)
'vehicle borne by horse' is the wordplay.
'vehicle' becomes 'cab' (cab is a kind of vehicle).
'borne by' is an insertion indicator (to bear can mean to carry).
'horse' becomes 'mare' (mare is a kind of horse).
'cab' placed within 'mare' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "'Grisly, gruesome (7)'" , "Ghostly appearance" , "Gruesome, suggesting death" , "'Sinister, gruesome (7)'" , "Grotesque" .)