Crossword Genius

Good news for investor? Buy in quantity! (5,2)

Ross

I believe the answer is:

stock up

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

'buy in quantity' is the definition.
(I've seen this in another clue)

'good news for investor?' is the wordplay.
I cannot quite see how this works, but
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Times.
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