Snacks we should understand first (8)

I believe the answer is:

twiglets

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'snacks we should' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'understand first' is the wordplay.
'understand' becomes 'twig' (I've seen this before).
'first' becomes 'lets' (I can't explain this - if you can you should give a lot more credence to this answer).
'twig'+'lets'='TWIGLETS'

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I've seen this clue in The Independent.

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