Harold only has a saintly sign (4)
I believe the answer is:
halo
'a saintly sign' is the definition.
(I've seen this in another clue)
'harold only' is the wordplay.
'harold' becomes 'Hal' (diminutive, as in Hal Prince).
'only' becomes 'o' (this could be a standard abbreviation I've not previously seen).
'hal'+'o'='HALO'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for halo that I've seen before include "Circle over a saint" , "Hagiological aura featured in Christian art" , "Light around sun, saint" , "Sign of a saint" , "Ring round head of saint" .)