One of Bart Simpson's sisters (4)
I believe the answer is:
lisa
(Other definitions for lisa that I've seen before include "Most intelligent of Marge Simpson's children" , "Leonardo da Vinci painted the 'Mona ....'" , "Mona . . . ., famous painting by Leonardo da Vinci" , "Girl" , "Mona ..., also called La Gioconda" .)