Crossword Genius

A mini Tesco mistakenly knocking point off bill for bag for life (8,3)

Ross

I believe the answer is:

amniotic sac

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

'a mini tesco mistakenly knocking point off bill' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'bag for life' is the wordplay.
I cannot really see how this works, but
'bag' could be 'sac' (a small bag) and 'sac' is present in the answer.
The remaining letters 'amniotic' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'for' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.
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