Spelling expert loses face about unknown, archaic letter (6)

I believe the answer is:

izzard

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

'letter' is the definition.
(I know that izzard is a type of letter)

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

(Other definitions for izzard that I've seen before include "Comedian" , "Comedy actor" .)

I've seen this clue in The Times.

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