Grim horse taking taxi (7)
I believe the answer is:
macabre
'grim' is the definition.
(I know that grim can be written as macabre)
'horse taking taxi' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse).
'taking' means one lot of letters goes inside another.
'taxi' becomes 'cab' (synonyms).
'mare' placed around 'cab' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Brace ma, being gruesome" , "Chilling" , "'Grisly, gruesome (7)'" , "Dark and disturbing" , "Gruesome, morbid" .)