Get jammy US extract (phosphorus-free) (4,3)

I believe the answer is:

luck out

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'get jammy us extract' is the definition.
I can't judge whether this defines the answer.

'phosphorus-free' is the wordplay.
'phosphorus' becomes 'luck' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'free' becomes 'out' (I've seen this before).
'luck'+'out'='LUCK OUT'

Can you help me to learn more?

(Another definition for luck out that I've seen is " win, by chance".)

I've seen this clue in The Times and The Sunday Times.

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