A criminal casing back of flat in London district (5)
I believe the answer is:
acton
'london district' is the definition.
(I've seen this before)
'a criminal casing back of flat' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'casing' means one lot of letters goes inside another (I've seen this in other clues).
'back of' indicates one should take the final letters.
The final letter of 'flat' is 't'.
'con' going around 't' is 'cton'.
'a'+'cton'='ACTON'
'in' acts as a link.
(Other definitions for acton that I've seen before include "English statesman" , "Part of the London borough of Ealing - British historian, d. 1902" , "London area" , "Gladstone's adviser" , "part of London on the tube" .)