The girl who can exist on nothing? (5)
I believe the answer is:
olive
'the girl' is the definition.
'exist on nothing?' is the wordplay.
'exist' becomes 'live' ('live' can be a synonym of 'exist').
'on' means one lot of letters go next to another.
'nothing?' becomes 'o' (looks like zero - 0).
'live' put after 'o' is 'OLIVE'.
'who can' acts as a link.
(Other definitions for olive that I've seen before include "Yellowy-green colour and fruit" , "Evergreen Mediterranean tree" , "Fruit, source of oil" , "Yellow-green colour like Mediterranean fruit" , "Mediterranean fruit or greenish colour" .)