Security firm, after a while in American city, gets on top of the legislation (10)
I believe the answer is:
collateral
'security' is the definition.
(collateral is a kind of security)
'firm after a while in american city gets on top of the legislation' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'after' means one lot of letters go next to another.
'a while in american city' becomes 'lateral' (I am not sure about this - if you are sure you should believe this answer much more).
'gets' indicates putting letters inside (gets can mean captures or absorbs).
'on top of' says to take the initial letters (I've seen 'on top' mean this).
The initial letter of 'legislation' is 'l'.
'lateral' placed around 'l' is 'llateral'.
'co'+'llateral'='COLLATERAL'
Can you help me to learn more?
(Other definitions for collateral that I've seen before include "running side by side" , "damage?" , "Confirmatory - recall a lot (anag)" , "Money put up" , "subsidiary" .)