Sick equine holding carriage (7)
I believe the answer is:
macabre
'sick' is the definition.
(thesaurus)
'equine holding carriage' is the wordplay.
'equine' becomes 'mare' (mare is a kind of equine).
'holding' is an insertion indicator.
'carriage' becomes 'cab' (cab is a kind of carriage).
'mare' going around 'cab' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Chilling" , "Ghostly appearance" , "Gruesome and weird" , "Ghastly, gruesome" , "Brace ma, being gruesome" .)