Starts to hear about lad's onyx ring (4)
I believe the answer is:
halo
'ring' is the definition.
('halo' can be a synonym of 'ring')
'starts to hear about lad's onyx' is the wordplay.
'starts to' indicates taking the first letters.
The initial letters of 'hear about lads onyx' is 'HALO'.
(Other definitions for halo that I've seen before include "Ring round head of saint" , "Circle of light round the sun or moon" , "Sign of a saint" , "Band of light" , "Light around sun, saint" .)