Terrible law-breaker having endless fun! (5)
I believe the answer is:
awful
'terrible' is the definition.
(synonyms)
'law-breaker having endless fun' is the wordplay.
'law breaker' becomes 'awl' (I can't explain this - if you can you should give a lot more credence to this answer).
'having' is an insertion indicator.
'endless' means to remove the last letter.
'fun' with its last letter taken off is 'fu'.
'awl' going around 'fu' is 'AWFUL'.
Can you help me to learn more?
(Other definitions for awful that I've seen before include "The pits" , "Appalling, terrible" , "Horrific" , "Dreadful, really bad" , "shocking" .)