Harold has nothing but light (4)
I believe the answer is:
halo
'light' is the definition.
(I know that halo is a type of light)
'harold has nothing' is the wordplay.
'harold' becomes 'Hal' (diminutive, as in Hal Prince).
'has' means one lot of letters go next to another.
'nothing' becomes 'O' (looks like zero - 0).
'hal'+'o'='HALO'
'but' is the link.
(Other definitions for halo that I've seen before include "Aura round admired person" , "Ring of saintliness" , "glory" , "Aura representing saintliness" , "Saint's circle of light" .)