California city began to collapse into lake (4,5)
I believe the answer is:
long beach
'california city' is the definition.
(city in Southern California)
'began to collapse into lake' is the wordplay.
'to collapse' indicates an anagram.
'into' means one lot of letters goes inside another.
'lake' becomes 'loch' (loch is a kind of lake).
'began' with letters rearranged gives 'ngbea'.
'ngbea' placed within 'loch' is 'LONG BEACH'.
(Other definitions for long beach that I've seen before include "American resort?" , "US city" , "Coastal resort" , "Chesil Bank, for instance" , "US naval base" .)