Mineral, only without large cross (4)
I believe the answer is:
Here is my best explanation:
'mineral' is the definition.
(onyx is a kind of mineral)
'only without large cross' is the wordplay.
'without' indicates named letters should be taken away.
'large' becomes 'l'.
'cross' becomes 'x'.
'only' with 'l' removed is 'ony'.
(Other definitions for onyx that I've seen before include "Chalcedony agate", "form of agate", "Agate used in cameos", "Smooth stone with bands of colour", "African antelope", "Semi-precious variety of agate", "Stone-like marble with coloured layers", "A semi-precious agate", "A semi-precious stone" and "precious stone".)