Permissible to go topless? That's terrible (5)
I believe the answer is:
awful
'terrible' is the definition.
('awful' can be a synonym of 'terrible')
'permissible to go topless?' is the wordplay.
'permissible' becomes 'lawful' (I've seen this before).
'to go topless?' means to remove the first letter (I've seen 'go topless' mean this).
'lawful' with its initial letter taken off is 'AWFUL'.
'that's' acts as a link.
(Other definitions for awful that I've seen before include "Dreadful, really bad" , "pants" , "dire result" , "Disgusting" , "Dreadful and dire" .)