Harold with nothing but a sign of holiness (4)
I believe the answer is:
halo
'a sign of holiness' is the definition.
(I've seen this in another clue)
'harold with nothing' is the wordplay.
'harold' becomes 'Hal' (diminutive, as in Hal Prince).
'with' is a charade indicator (letters next to each other).
'nothing' becomes 'O' (looks like zero - 0).
'hal'+'o'='HALO'
'but' acts as a link.
(Other definitions for halo that I've seen before include "Corona" , "Saint's circle of light" , "Ring of light (signifying holiness)" , "Saint's overhead ring of light" , "Circle over saint's head in painting" .)