A letter and its addendum I added (3)

I believe the answer is:

psi

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

I can't explain the rest of the clue.

(Other definitions for psi that I've seen before include "one employed by Socrates?" , "Pounds per sq. in. (abbr.)" , "foreign letter" , "Greek character" , "letter from abroad" .)

I've seen this clue in The Independent.

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