Crossword Genius

When pictures are getting more pricey, buys quickly (5,2)

Ross

I believe the answer is:

snaps up

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'when pictures' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how one could define the other.

I can't explain the rest of the clue.

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I've seen this clue in the Evening Standard.
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