Taxi runs into horse - gruesome! (7)
I believe the answer is:
macabre
'gruesome' is the definition.
(both can mean morbid or disturbing)
'taxi runs into horse' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'runs into' is an insertion indicator.
'horse' becomes 'mare' (mare is a kind of horse).
'cab' inserted inside 'mare' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Brace ma, being gruesome" , "gruesome result" , "Gruesome, morbid" , "Gruesome, suggesting death" , "Morbidly repellent" .)