Ugly but allowed to go topless (5)
I believe the answer is:
awful
'ugly' is the definition.
(thesaurus)
'allowed to go topless' is the wordplay.
'allowed' becomes 'lawful' (I've seen this before).
'to go topless' means to remove the first letter.
'lawful' with its initial letter removed is 'AWFUL'.
'but' acts as a link.
(Other definitions for awful that I've seen before include "Horrible" , "Dreadful, really bad" , "Very poor" , "Exceptionally bad" , "Very embarrassing" .)