Rubbish bags for geological feature (3,4)

I believe the answer is:

cap rock

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'rubbish' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see a link between them that I can't see?

'for geological feature' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'geological' becomes 'cack' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'feature' indicates putting letters inside.
'pro' put into 'cack' is 'CAP ROCK'.

'bags' acts as a link.

Can you help me to learn more?

I've seen this clue in The Independent.

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