Caesar's heart found in morgue, horribly macabre (8)
I believe the answer is:
gruesome
'macabre' is the definition.
(I've seen this before)
'caesar's heart found in morgue horribly' is the wordplay.
'heart' says to take the centre.
'found in' indicates putting letters inside.
'horribly' indicates anagramming the letters.
The central letters of 'caesar' are 'es'.
'morgue' is an anagram of 'gruome'.
'es' going into 'gruome' is 'GRUESOME'.
(Other definitions for gruesome that I've seen before include "Repulsive, horrid" , "Shockingly repellent; inspiring horror" , "Revolting, filling one with horror and disgust" , "'Grisly, macabre (8)'" , "'Ghastly, grisly (8)'" .)