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 subsidiary indication.
'without' suggests deleting specific letters.
'large' becomes 'l'.
'cross' becomes 'x'.
'only' with 'l' taken away is 'ony'.
(Other definitions for onyx that I've seen before include "African antelope", "Banded gemstone", "Mineral with coloured layers", "Sort of agate", "A form of quartz", "Agate used in cameos", "Smooth stone with bands of colour", "A semi-precious stone", "Marble-like gemstone" and "Form of quartz".)