Footnote I add to a letter from Greece (3)
I believe the answer is:
psi
'letter from greece' is the definition.
(letter from the Greek alphabet)
'footnote i add' is the wordplay.
I cannot really understand how this works, but
'footnote' could be 'ps' and 'ps' is found in the answer.
'i' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'to a' acts as a link.
Can you help me to learn more?
(Other definitions for psi that I've seen before include "Greek letter; pounds per square inch (abbrev.)" , "letter from abroad" , "Twenty-third letter of Greek alphabet" , "Pounds per sq. in. (abbr.)" , "one employed by Socrates?" .)