Protecting nothing, Thatcher set out to be ruinous (4,3,5)

I believe the answer is:

cost the earth

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'be ruinous' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'protecting nothing thatcher set out' is the wordplay.
'protecting' indicates putting letters inside (inserted letters are held or protected).
'nothing' becomes 'O' (looks like zero - 0).
'out' indicates an anagram (out can mean wrong or inaccurate).
'thatcher'+'set'='thatcherset'
'thatcherset' anagrammed gives 'csttheearth'.
'o' placed inside 'csttheearth' is 'COST THE EARTH'.

'to' is the link.

Can you help me to learn more?

(Another definition for cost the earth that I've seen is " proved extremely expensive".)

I've seen this clue in The Guardian.

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