Form of chalcedony (4)
I believe the answer is:
'form of chalcedony' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for onyx that I've seen before include "Semi-precious variety of agate" , "Type of agate" , "Marbled stone with layers of colours" , "Black variety of quartz" , "Smooth stone with bands of colour" .)