Why you need proof, of course (2,2,4)

I believe the answer is:

to be sure

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'of course' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for to be sure that I've seen before include "Undoubtedly" , "Without doubt" , "Admittedly" , "Of course, certainly" , "I admit" .)

I've seen this clue in the Evening Standard.

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