Next-to-last letter (3)
I believe the answer is:
psi
'next-to-last letter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for psi that I've seen before include "Twenty-third letter of Greek alphabet" , "Greek character" , "Greek letter; pressure unit" , "letter from abroad" , "Pounds per sq. in. (abbr.)" .)