Taxi running into horse -- gruesome! (7)
I believe the answer is:
macabre
'gruesome' is the definition.
(both can mean morbid or disturbing)
'taxi running into horse' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'running into' indicates putting letters inside.
'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 "gruesome result" , "Gruesome and weird" , "'Grim, repellent (7)'" , "Grotesque" , "Ghastly, gruesome" .)