What almost destroyed west London district (11)
I believe the answer is:
walthamstow
'london district' is the definition.
'what almost destroyed west' is the wordplay.
'destroyed' indicates an anagram.
'west' becomes 'w' ('W' can be a synonym of 'west').
'what'+'almost'='whatalmost'
'whatalmost' anagrammed gives 'walthamsto'.
'walthamsto'+'w'='WALTHAMSTOW'
(Other definitions for walthamstow that I've seen before include "London area" , "Gtr London town" .)