Footnote I added to a letter from Greece (3)
I believe the answer is:
psi
'a letter from greece' is the definition.
(letter from the Greek alphabet)
'footnote i added' is the wordplay.
'footnote' becomes 'PS'.
'added' says to put letters next to each other.
'ps'+'i'='PSI'
'to' acts as a link.
(Other definitions for psi that I've seen before include "Pounds per square inch (abbr.); Greek letter" , "foreign letter" , "Pounds per sq. in. (abbr.)" , "one employed by Socrates?" , "Greek character" .)