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 wordplay.
'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 "Black and white striped gemstone", "A semi-precious stone", "Mineral with coloured layers", "Form of quartz", "Stones", "Marbled stone with layers of colours", "Quartz used as a gem", "Gemstone, a variety of chalcedony", "Banded chalcedony" and "Semi-precious variety of agate".)