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