Harold has nothing but ring (4)
I believe the answer is:
halo
'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 "Saintly ring" , "Saint's circle of light" , "Circle of light around the moon" , "Corona" , "Hagiological aura featured in Christian art" .)