Cause of extreme weather faked online (2,4)
I believe the answer is:
el nino
'cause of extreme weather' is the definition.
(I've seen this in another clue)
'faked online' is the wordplay.
'faked' indicates an anagram (I've seen 'faking' mean this).
'online' anagrammed gives 'EL NINO'.
(Other definitions for el nino that I've seen before include "Periodic warming of the eastern Pacific Ocean" , "Cause of weather change" , "Periodic climatic phenomenon" , "Pacific climatic effect" , "Pacific weather feature" .)