Security arrest gangster casing empty theatre (10)
I believe the answer is:
collateral
'security' is the definition.
(collateral is a kind of security)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for collateral that I've seen before include "Aside from the main issue" , "subsidiary" , "Confirmatory - recall a lot (anag)" , "Secondary; parallel" , "damage?" .)