Shun heavy alternative to olive oil (4,3)

I believe the answer is:

duck fat

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'shun heavy alternative' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'olive oil' is the wordplay.
'olive' becomes 'duck' (I can't explain this - if you can you should give a lot more credence to this answer).
'oil' becomes 'fat'.
'duck'+'fat'='DUCK FAT'

'to' is the link.

Can you help me to learn more?

I've seen this clue in The Guardian.

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