Crazy girl I dumped (3)
I believe the answer is:
mad
'crazy' is the definition.
(I know that crazy can be written as mad)
I don't understand how the rest of the clue works.
(Other definitions for mad that I've seen before include "Insane - furious" , "Crazed" , "Aberrant" , "Crazy" , "Deranged" .)