A bleared vision probable isn't (8)

I believe the answer is:

readable

I'm a little stuck... Click here to teach me more about this clue!

'vision probable isn't' is the definition.
I can't tell whether this definition defines the answer.

'a bleared' is the wordplay.
'a bleared' anagrams to 'READABLE'.
Though, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for readable that I've seen before include "okay as literature" , "can be studied" , "Legible" , "Describing text" , "eye can interpret" .)

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

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