Horrible beheading initially caught by mobile camera (7)
I believe the answer is:
Here is my best explanation:
'horrible' is the definition.
'beheading initially caught by mobile camera' is the wordplay.
'initially' says to take the initial letters.
'caught by' indicates putting letters inside.
'mobile' indicates an anagram.
The initial letter of 'beheading' is 'b'.
'camera' with letters rearranged gives 'macare'.
'b' going within 'macare' is 'MACABRE'.
(Other definitions for macabre that I've seen before include "Ghostly appearance", "Brace ma, being gruesome", "Gruesome, suggesting death", "'Grim, repellent (7)'", "Gruesome and weird", "Shockingly horrible", "'Grisly, gruesome (7)'", "Chilling", "Dark and disturbing" and "Disturbing".)