How did you do this?
first make sure you understand that you shouldnt be exposing a private key to a address that still hold coins, so understand it and dtake the appropriate measures.
on multibit go tools -> export private keys
select "do not password protect" so you can read the keys in plaintext, then export it to a file, read the file in a text editor and copy the private key(s)
on CLAM go to help -> debug window -> console
importprivkey x (where x is the private key you are importing)
restart clam and launch the wallet with the -salvagewallet argument