Beheading legal? That's disgusting (5)
I believe the answer is:
awful
'disgusting' is the definition.
(I've seen this before)
'beheading legal?' is the wordplay.
'beheading' means to remove the first letter.
'legal?' becomes 'lawful' ('lawful' is associated in meaning with 'legal').
'lawful' with its initial letter removed is 'AWFUL'.
'that's' is the link.
(Other definitions for awful that I've seen before include "Dreadful, really bad" , "Dreadful and dire" , "pants" , "Extremely bad or unpleasant" , "Horrific" .)