Ring involved in law-breaking is missing (4)
I believe the answer is:
awol
'is missing' is the definition.
(military abbreviation for absent without leave)
'ring involved in law-breaking' is the wordplay.
'ring' becomes 'o' (letter resembling a ring).
'involved in' is an insertion indicator.
'breaking' indicates an anagram.
'law' is an anagram of 'awl'.
'o' put within 'awl' is 'AWOL'.
(Other definitions for awol that I've seen before include "unaccounted for" , "Absent without permission, in short" , "Of soldier, illegally absent - initially" , "Truant (or just missing)" , "On the lam" .)