Landslide victory gained by one in a Chanel ensemble (9)
I believe the answer is:
Here is my best explanation:
'landslide' is the definition.
(I've seen this before)
'victory gained by one in a chanel ensemble' is the wordplay.
'victory' becomes 'v'.
'gained by' becomes 'aa' (I can't justify this - if you can you should believe this answer much more).
'one in' is an insertion indicator.
'ensemble' indicates an anagram.
'v' placed into 'aa' is 'ava'.
'chanel' is an anagram of 'lanche'.