Without vetting ultimately, possibly guessing then? (5,6)

I believe the answer is:

sight unseen

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But I can't explain it at all! Could you help me understand?

(Other definitions for sight unseen that I've seen before include "without prior inspection" , "How one may buy" , "something missed by tourist?" , "as this puzzle is done" .)

I've seen this clue in The Times.

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