Control supply of rocks that are important in building (12)

I believe the answer is:

cornerstones

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'that are important in building' is the definition.
I can't tell whether this definition defines the answer.

'control supply of rocks' is the wordplay.
'control' becomes 'corner' (cornering is a kind of controlling).
'supply of rocks' becomes 'stones' (I can't explain this - if you can you should believe this answer much more).
'corner'+'stones'='CORNERSTONES'

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I've seen this clue in The Globe and Mail.

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