Stone, only without large cross (4)
I believe the answer is:
Here is my best explanation:
'stone' is the definition.
(I've seen this before)
'only without large cross' is the subsidiary indication.
'without' suggests deleting specific letters.
'large' becomes 'l'.
'cross' becomes 'x'.
'only' with 'l' removed is 'ony'.
(Other definitions for onyx that I've seen before include "Cameo material", "Form of chalcedony", "Marble-like gemstone", "Semi-precious stone used in making cameos", "Agate with different bands of colour", "Black and white striped gemstone", "Marbled stone with layers of colours", "Gemstone, a variety of chalcedony", "Mineral with coloured layers" and "African antelope".)