You and I have nothing but sadness (3)
I believe the answer is:
woe
'sadness' is the definition.
(woe is a kind of sadness)
'you and i have nothing' is the wordplay.
'you and i' becomes 'we' (we means you and I).
'have' indicates putting letters inside.
'nothing' becomes 'O' (O resembles 0).
'we' going around 'o' is 'WOE'.
'but' acts as a link.
(Other definitions for woe that I've seen before include "Tale content?" , "Misfortune" , "'Wednesday's child is full of ...'" , "Misery - tribulation" , "Archaic sorrow or distress" .)