Harold has nothing but a ring (4)
I believe the answer is:
halo
'a ring' is the definition.
(I know that ring can be written as halo)
'harold has nothing' is the wordplay.
'harold' becomes 'Hal' (diminutive, as in Hal Prince).
'has' says to put letters next to each other.
'nothing' becomes 'O' (looks like zero - 0).
'hal'+'o'='HALO'
'but' is the link.
(Other definitions for halo that I've seen before include "Sign of sainthood" , "Circle painted over saint's head" , "Glittering aura" , "Hagiological aura featured in Christian art" , "Ring round head of saint" .)