Terrible law-breaker having endless fun! (5)
I believe the answer is:
awful
'terrible' is the definition.
('awful' can be a synonym of 'terrible')
'law-breaker having endless fun' is the wordplay.
'law breaker' becomes 'awl' (I can't justify this - if you can you should give a lot more credence to this answer).
'having' means one lot of letters goes inside another.
'endless' means to remove the last letter.
'fun' with its final letter removed is 'fu'.
'awl' placed 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" .)