Bust ringleader imprisoned by magistrate (5)
I believe the answer is:
break
'bust' is the definition.
('break' can be a synonym of 'bust')
'ringleader imprisoned by magistrate' is the wordplay.
'ringleader' becomes 'r' (first letter of ring).
'imprisoned by' means one lot of letters goes inside another.
'magistrate' becomes 'beak' (I've seen this before).
'r' put inside 'beak' is 'BREAK'.
(Other definitions for break that I've seen before include "Smash" , "Shatter; respite" , "Time off working" , "Holiday; snap" , "Come to an end" .)