In low spirits because stock varieties gone? (10)

'in low spirits because stock' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'varieties gone?' is the wordplay.
I cannot really see how this works, but
'varieties' could be 'sorts' ('sort' can be a synonym of 'variety') and 'sorts' is located in the answer.
'gone?' could be 'out' ('out' can be a synonym of 'gone') and 'out' is present in the answer.
The remaining letters 'of' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?