Horrible beheading initially caught by mobile camera (7)
I believe the answer is:
macabre
'horrible' is the definition.
(thesaurus)
'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 "Morbidly repellent" , "Shockingly horrible" , "Gruesome, suggesting death" , "Brace ma, being gruesome" , "Ghastly, gruesome" .)