Make vague promo for jacket with no back (4)

I believe the answer is:

blur

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'make vague' is the definition.
(synonyms)

I can't explain the rest of the clue.

(Other definitions for blur that I've seen before include "Hazy patch" , "fade" , "Obscure; smudge" , "Smear, make indistinct" , "Group - thing not clearly seen" .)

I've seen this clue in The Guardian.

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