Taxi swallowed by horse? Gruesome! (7)
I believe the answer is:
macabre
'gruesome' is the definition.
(both can mean morbid or disturbing)
'taxi swallowed by horse?' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'swallowed by' is an insertion indicator.
'horse?' becomes 'mare' (mare is a kind of horse).
'cab' inserted within 'mare' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Dark and disturbing" , "Gruesome, suggesting death" , "Chilling" , "'Grisly, gruesome (7)'" , "gruesome result" .)