Legal to be beheaded? That's terrible (5)
I believe the answer is:
awful
'terrible' is the definition.
(I know that terrible can be written as awful)
'legal to be beheaded?' is the wordplay.
'legal' becomes 'lawful' (associated in meaning).
'to be beheaded?' means to remove the first letter (the head of the word is removed).
'lawful' with its first letter taken away is 'AWFUL'.
'that's' is the link.
(Other definitions for awful that I've seen before include "Horrible" , "Dreadful, really bad" , "Dreadful - spine-chilling" , "Not good at all" , "Dreadful, nasty" .)