Maggie Simpson's sister (4)
I believe the answer is:
lisa
'maggie simpson'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 "Mona . . . ., famous painting by Leonardo da Vinci" , "Girl" , "Most intelligent of Marge Simpson's children" , "Leonardo da Vinci painted the 'Mona ....'" , "Mona ..., also called La Gioconda" .)