Legal to be guillotined, though dreadful (5)
I believe the answer is:
awful
'dreadful' is the definition.
('awful' can be a synonym of 'dreadful')
'legal to be guillotined' is the wordplay.
'legal' becomes 'lawful' ('lawful' is associated in meaning with 'legal').
'to be guillotined' means to remove the first letter.
'lawful' with its initial letter removed is 'AWFUL'.
'though' acts as a link.
(Other definitions for awful that I've seen before include "Extremely bad or unpleasant" , "Disgusting" , "The pits" , "Really terrible, deplorable" , "Fearsome" .)