JFK letters (3)
I believe the answer is:
eta
'jfk letters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eta that I've seen before include "Foreign letter" , "Greek letter - Basque nationalist movement" , "Basque separatist movement" , "Something like 10 [RHO]" , "Greek letter E; estimated time of arrival (abbr.)" .)