Letter, or what’s added to it by one (3)
I believe the answer is:
psi
'letter' is the definition.
(I know that psi is a type of letter)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for psi that I've seen before include "Greek character" , "Pounds per sq. in. (abbr.)" , "Greek letter; pressure unit" , "letter from abroad" , "Twenty-third letter of Greek alphabet" .)