Gruesome, when taxi goes into horse (7)
I believe the answer is:
macabre
'gruesome' is the definition.
(synonyms)
'taxi goes into horse' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'goes into' indicates putting letters inside.
'horse' becomes 'mare' (mare is a kind of horse).
'cab' inserted within 'mare' is 'MACABRE'.
'when' is the link.
(Other definitions for macabre that I've seen before include "Dark and disturbing" , "Gruesome and weird" , "'Grisly, gruesome (7)'" , "'Sinister, gruesome (7)'" , "Gruesome, suggesting death" .)