Law about British billions remains (6)
I believe the answer is:
rubble
'remains' is the definition.
(I've seen this before)
'law about british billions' is the wordplay.
'law' becomes 'rule' (rule is a kind of law).
'about' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'billions' becomes 'b'.
'b'+'b'='bb'
'rule' enclosing 'bb' is 'RUBBLE'.
(Other definitions for rubble that I've seen before include "Waste fragments of stone or rock, debris" , "Demolition site debris" , "Light theatrical show" , "Remains of demolished building" , "rubbish?" .)