Quantity of market stock in agricultural machinery? (12)

I believe the answer is:

ploughshares

I'm a little stuck... Click here to teach me more about this clue!

'quantity of market stock in agricultural machinery?' is the definition.
I can't tell whether this defines the answer.

'stock in agricultural machinery?' is the wordplay.
'stock' becomes 'share' (share is a kind of stock).
'in' means one lot of letters goes inside another.
'agricultural machinery?' becomes 'ploughs' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'share' put inside 'ploughs' is 'PLOUGHSHARES'.

Can you help me to learn more?

I've seen this clue in The Irish News.

Want a hint initially instead of a full solution? Install my app