Bart's sister (4)
I believe the answer is:
lisa
'bart's sister?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lisa that I've seen before include "Most intelligent of Marge Simpson's children" , "Leonardo da Vinci painted the 'Mona ....'" , "Mona ..., also called La Gioconda" , "Girl" , "Mona . . . ., famous painting by Leonardo da Vinci" .)