Carriage cutting horse in two: it's ghastly (7)
I believe the answer is:
macabre
'ghastly' is the definition.
(I know that ghastly can be written as macabre)
'carriage cutting horse' is the wordplay.
'carriage' becomes 'cab' (cab is a kind of carriage).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'horse' becomes 'mare' (mare is a kind of horse).
'cab' placed into 'mare' is 'MACABRE'.
'in two it's' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for macabre that I've seen before include "Morbidly repellent" , "Shockingly horrible" , "'Grisly, gruesome (7)'" , "Ghostly appearance" , "'Grim, repellent (7)'" .)