Lacking home cover for certain (4)

I believe the answer is:

sure

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'certain' is the definition.
(I know that certain can be written as sure)

I don't understand the remainder of the clue.

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(Other definitions for sure that I've seen before include "Confident sort of ruse" , "Certain to be a ruse" , "certainly in the USA" , "Definite" , "stable" .)

I've seen this clue in The Guardian.

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