Legal to be beheaded? Terrible! (5)
I believe the answer is:
awful
'terrible' is the definition.
(synonyms)
'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 initial letter taken off is 'AWFUL'.
(Other definitions for awful that I've seen before include "Extremely bad or unpleasant" , "Very poor" , "Atrocious, dire" , "Not good at all" , "Appalling, terrible" .)