Marx Brother wants nothing smart (4)
I believe the answer is:
chic
'smart' is the definition.
(both can mean stylish)
'marx brother wants nothing' is the wordplay.
'marx brother' becomes 'chico' (I've seen this in another clue).
'wants' indicates named letters should be taken away ('want' can mean 'lack').
'nothing' becomes 'O' (O resembles 0).
'chico' with 'o' removed is 'CHIC'.
(Other definitions for chic that I've seen before include "Unwanted sounds" , "Fashionably smart" , "Elegant and stylishly fashionable" , "Stylishly turned out" , "Elegantly fashionable" .)